lib: bitmap: remove unnecessary mask from bitmap_complement

Since the extra bits are "don't care", there is no reason to mask the
last word to the used bits when complementing.  This shaves off yet a
few bytes.

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>