Rewrite function-like macro into regular static inline function to
avoid a warning during macro expansion.

Fix warning (treated as error in W=1):
./arch/powerpc/include/asm/uaccess.h:52:35: error: comparison of unsigned expression >= 0 is always true
   (((size) == 0) || (((size) - 1) <= ((segment).seg - (addr)))))
                                   ^
Suggested-by: NSegher Boessenkool <segher@kernel.crashing.org>
Signed-off-by: NMathieu Malaterre <malat@debian.org>
Reviewed-by: NChristophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>