[PATCH] x86-64: bitops fix for -Os

This fixes the x86-64 find_[first|next]_zero_bit() function for the
end-of-range case.  It didn't test for a zero size, and the "rep scas"
would do entirely the wrong thing.

Signed-off-by: Alexandre Oliva <oliva@lsd.ic.unicamp.br>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>