[PATCH] mm: do not check unpopulated zones for draining and counter updates

If a zone is unpopulated then we do not need to check for pages that are to
be drained and also not for vm counters that may need to be updated.

Signed-off-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>