mempool: add unlikely and likely hints

Add unlikely and likely hints to the function mempool_free.  It lays out
the code in such a way that the common path is executed straighforward and
saves a cache line.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>