mm could be removed from current task struct, using previous vma->vm_mm

It will crash on blackfin after updated to Linux 3.15.  The commit "mm:
per-thread vma caching" caused the crash.  mm could be removed from
current task struct before

  mmput()->
    exit_mmap()->
      delete_vma_from_mm()

the detailed fault information:

    NULL pointer access
    Kernel OOPS in progress
    Deferred Exception context
    CURRENT PROCESS:
    COMM=modprobe PID=278  CPU=0
    invalid mm
    return address: [0x000531de]; contents of:
    0x000531b0:  c727  acea  0c42  181d  0000  0000  0000  a0a8
    0x000531c0:  b090  acaa  0c42  1806  0000  0000  0000  a0e8
    0x000531d0:  b0d0  e801  0000  05b3  0010  e522  0046 [a090]
    0x000531e0:  6408  b090  0c00  17cc  3042  e3ff  f37b  2fc8

    CPU: 0 PID: 278 Comm: modprobe Not tainted 3.15.0-ADI-2014R1-pre-00345-gea9f446 #25
    task: 0572b720 ti: 0569e000 task.ti: 0569e000
    Compiled for cpu family 0x27fe (Rev 0), but running on:0x0000 (Rev 0)
    ADSP-BF609-0.0 500(MHz CCLK) 125(MHz SCLK) (mpu off)
    Linux version 3.15.0-ADI-2014R1-pre-00345-gea9f446 (steven@steven-OptiPlex-390) (gcc version 4.3.5 (ADI-trunk/svn-5962) ) #25 Tue Jun 10 17:47:46 CST 2014

    SEQUENCER STATUS:		Not tainted
     SEQSTAT: 00000027  IPEND: 8008  IMASK: ffff  SYSCFG: 2806
      EXCAUSE   : 0x27
      physical IVG3 asserted : <0xffa00744> { _trap + 0x0 }
      physical IVG15 asserted : <0xffa00d68> { _evt_system_call + 0x0 }
      logical irq   6 mapped  : <0xffa003bc> { _bfin_coretmr_interrupt + 0x0 }
      logical irq   7 mapped  : <0x00008828> { _bfin_fault_routine + 0x0 }
      logical irq  11 mapped  : <0x00007724> { _l2_ecc_err + 0x0 }
      logical irq  13 mapped  : <0x00008828> { _bfin_fault_routine + 0x0 }
      logical irq  39 mapped  : <0x00150788> { _bfin_twi_interrupt_entry + 0x0 }
      logical irq  40 mapped  : <0x00150788> { _bfin_twi_interrupt_entry + 0x0 }
     RETE: <0x00000000> /* Maybe null pointer? */
     RETN: <0x0569fe50> /* kernel dynamic memory (maybe user-space) */
     RETX: <0x00000480> /* Maybe fixed code section */
     RETS: <0x00053384> { _exit_mmap + 0x28 }
     PC  : <0x000531de> { _delete_vma_from_mm + 0x92 }
    DCPLB_FAULT_ADDR: <0x00000008> /* Maybe null pointer? */
    ICPLB_FAULT_ADDR: <0x000531de> { _delete_vma_from_mm + 0x92 }
    PROCESSOR STATE:
     R0 : 00000004    R1 : 0569e000    R2 : 00bf3db4    R3 : 00000000
     R4 : 057f9800    R5 : 00000001    R6 : 0569ddd0    R7 : 0572b720
     P0 : 0572b854    P1 : 00000004    P2 : 00000000    P3 : 0569dda0
     P4 : 0572b720    P5 : 0566c368    FP : 0569fe5c    SP : 0569fd74
     LB0: 057f523f    LT0: 057f523e    LC0: 00000000
     LB1: 0005317c    LT1: 00053172    LC1: 00000002
     B0 : 00000000    L0 : 00000000    M0 : 0566f5bc    I0 : 00000000
     B1 : 00000000    L1 : 00000000    M1 : 00000000    I1 : ffffffff
     B2 : 00000001    L2 : 00000000    M2 : 00000000    I2 : 00000000
     B3 : 00000000    L3 : 00000000    M3 : 00000000    I3 : 057f8000
    A0.w: 00000000   A0.x: 00000000   A1.w: 00000000   A1.x: 00000000
    USP : 056ffcf8  ASTAT: 02003024

    Hardware Trace:
       0 Target : <0x00003fb8> { _trap_c + 0x0 }
         Source : <0xffa006d8> { _exception_to_level5 + 0xa0 } JUMP.L
       1 Target : <0xffa00638> { _exception_to_level5 + 0x0 }
         Source : <0xffa004f2> { _bfin_return_from_exception + 0x6 } RTX
       2 Target : <0xffa004ec> { _bfin_return_from_exception + 0x0 }
         Source : <0xffa00590> { _ex_trap_c + 0x70 } JUMP.S
       3 Target : <0xffa00520> { _ex_trap_c + 0x0 }
         Source : <0xffa0076e> { _trap + 0x2a } JUMP (P4)
       4 Target : <0xffa00744> { _trap + 0x0 }
          FAULT : <0x000531de> { _delete_vma_from_mm + 0x92 } P0 = W[P2 + 2]
         Source : <0x000531da> { _delete_vma_from_mm + 0x8e } P2 = [P4 + 0x18]
       5 Target : <0x000531da> { _delete_vma_from_mm + 0x8e }
         Source : <0x00053176> { _delete_vma_from_mm + 0x2a } IF CC JUMP pcrel
       6 Target : <0x0005314c> { _delete_vma_from_mm + 0x0 }
         Source : <0x00053380> { _exit_mmap + 0x24 } JUMP.L
       7 Target : <0x00053378> { _exit_mmap + 0x1c }
         Source : <0x00053394> { _exit_mmap + 0x38 } IF !CC JUMP pcrel (BP)
       8 Target : <0x00053390> { _exit_mmap + 0x34 }
         Source : <0xffa020e0> { __cond_resched + 0x20 } RTS
       9 Target : <0xffa020c0> { __cond_resched + 0x0 }
         Source : <0x0005338c> { _exit_mmap + 0x30 } JUMP.L
      10 Target : <0x0005338c> { _exit_mmap + 0x30 }
         Source : <0x0005333a> { _delete_vma + 0xb2 } RTS
      11 Target : <0x00053334> { _delete_vma + 0xac }
         Source : <0x0005507a> { _kmem_cache_free + 0xba } RTS
      12 Target : <0x00055068> { _kmem_cache_free + 0xa8 }
         Source : <0x0005505e> { _kmem_cache_free + 0x9e } IF !CC JUMP pcrel (BP)
      13 Target : <0x00055052> { _kmem_cache_free + 0x92 }
         Source : <0x0005501a> { _kmem_cache_free + 0x5a } IF CC JUMP pcrel
      14 Target : <0x00054ff4> { _kmem_cache_free + 0x34 }
         Source : <0x00054fce> { _kmem_cache_free + 0xe } IF CC JUMP pcrel (BP)
      15 Target : <0x00054fc0> { _kmem_cache_free + 0x0 }
         Source : <0x00053330> { _delete_vma + 0xa8 } JUMP.L
    Kernel Stack
    Stack info:
     SP: [0x0569ff24] <0x0569ff24> /* kernel dynamic memory (maybe user-space) */
     Memory from 0x0569ff20 to 056a0000
    0569ff20: 00000001 [04e8da5a] 00008000  00000000  00000000  056a0000  04e8da5a  04e8da5a
    0569ff40: 04eb9eea  ffa00dce  02003025  04ea09c5  057f523f  04ea09c4  057f523e  00000000
    0569ff60: 00000000  00000000  00000000  00000000  00000000  00000000  00000001  00000000
    0569ff80: 00000000  00000000  00000000  00000000  00000000  00000000  00000000  00000000
    0569ffa0: 0566f5bc  057f8000  057f8000  00000001  04ec0170  056ffcf8  056ffd04  057f9800
    0569ffc0: 04d1d498  057f9800  057f8fe4  057f8ef0  00000001  057f928c  00000001  00000001
    0569ffe0: 057f9800  00000000  00000008  00000007  00000001  00000001  00000001 <00002806>
    Return addresses in stack:
        address : <0x00002806> { _show_cpuinfo + 0x2d2 }
    Modules linked in:
    Kernel panic - not syncing: Kernel exception
    [ end Kernel panic - not syncing: Kernel exception

Signed-off-by: Steven Miao <realmz6@gmail.com>
Acked-by: Davidlohr Bueso <davidlohr@hp.com>
Reviewed-by: Rik van Riel <riel@redhat.com>
Acked-by: David Rientjes <rientjes@google.com>
Cc: <stable@vger.kernel.org>	[3.15.x]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>