m68k: Convert access_ok() to an inline function

Convert access_ok() from a macro to an inline function, so the compiler no
longer complains about unused variables:

    fs/read_write.c: In function 'rw_copy_check_uvector':
    fs/read_write.c:556: warning: unused variable 'buf'

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>