mm: unlockless reclaim

unlock_page is fairly expensive.  It can be avoided in page reclaim
success path.  By definition if we have any other references to the page
it would be a bug anyway.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>