lib: bitmap: micro-optimize bitmap_allocate_region

__reg_op(..., REG_OP_ALLOC) always returns 0, so we might as well use that
and save an instruction.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>