lib/find_bit.c: micro-optimise find_next_*_bit

This saves 32 bytes on my x86-64 build, mostly due to alignment
considerations and sharing more code between find_next_bit and
find_next_zero_bit, but it does save a couple of instructions.

There's really two parts to this commit:
 - First, the first half of the test: (!nbits || start >= nbits) is
   trivially a subset of the second half, since nbits and start are both
   unsigned
 - Second, while looking at the disassembly, I noticed that GCC was
   predicting the branch taken. Since this is a failure case, it's
   clearly the less likely of the two branches, so add an unlikely() to
   override GCC's heuristics.

[[email protected]: v2]
  Link: http://lkml.kernel.org/r/[email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Matthew Wilcox <[email protected]>
Acked-by: Yury Norov <[email protected]>
Acked-by: Rasmus Villemoes <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

tools/lib/find_bit.c

@@ -34,7 +34,7 @@ static unsigned long _find_next_bit(const unsigned long *addr,
 {
 	unsigned long tmp;
 
-	if (!nbits || start >= nbits)
+	if (unlikely(start >= nbits))
 		return nbits;
 
 	tmp = addr[start / BITS_PER_LONG] ^ invert;