The c->sup_node is allocated in function ubifs_read_sb_node but
is not freed. This will cause memory leak as below:

unreferenced object 0xbc9ce000 (size 4096):
  comm "mount", pid 500, jiffies 4294952946 (age 315.820s)
  hex dump (first 32 bytes):
    31 18 10 06 06 7b f1 11 02 00 00 00 00 00 00 00  1....{..........
    00 10 00 00 06 00 00 00 00 00 00 00 08 00 00 00  ................
  backtrace:
    [<d1c503cd>] ubifs_read_superblock+0x48/0xebc
    [<a20e14bd>] ubifs_mount+0x974/0x1420
    [<8589ecc3>] legacy_get_tree+0x2c/0x50
    [<5f1fb889>] vfs_get_tree+0x28/0xfc
    [<bbfc7939>] do_mount+0x4f8/0x748
    [<4151f538>] ksys_mount+0x78/0xa0
    [<d59910a9>] ret_fast_syscall+0x0/0x54
    [<1cc40005>] 0x7ea02790

Free it in ubifs_umount and in the error path of mount_ubifs.

Fixes: fd615005

 ("ubifs: Store read superblock node")
Signed-off-by: Quanyang Wang <quanyang.wang@windriver.com>
Signed-off-by: Richard Weinberger <richard@nod.at>