MIPS: use simpler access_ok()

Before unifying the mips version of __access_ok() with the generic
code, this converts it to the same algorithm. This is a change in
behavior on mips64, as now address in the user segment, the lower
2^62 bytes, is taken to be valid, relying on a page fault for
addresses that are within that segment but not valid on that CPU.

The new version should be the most effecient way to do this, but
it gets rid of the special handling for size=0 that most other
architectures ignore as well.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>