There are numerous places we check whether a zone is populated or not.

Provide a helper function to check for populated zones and convert all
checks for zone->present_pages.
Signed-off-by: NCon Kolivas <kernel@kolivas.org>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>