MIPS: Fix access_ok() for the last byte of user space

The MIPS implementation of access_ok() incorrectly reports that access
to the final byte of user memory is not OK, much as the alpha & SH
versions did prior to commit 94bd8a05 ("Fix 'acccess_ok()' on alpha
and SH").

For example on a MIPS64 system with __UA_LIMIT == 0xffff000000000000 we
incorrectly fail in the following cases:

  access_ok(0xffffffffffff, 0x1) = 0
  access_ok(0xfffffffffffe, 0x2) = 0

Fix MIPS in the same way as alpha & SH, by subtracting one from the addr
+ size condition when size is non-zero. With this the access_ok() calls
above return 1 indicating that the access may be valid.

The cost of the improved check is pretty minimal - we gain 2410 bytes,
or 0.03%, in kernel code size for a 64r6el_defconfig kernel built using
GCC 8.1.0.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>