mm: add zone nr_pages helper function

mm: add zone nr_pages helper function

Add zone_nr_pages() helper function.

It is used by a later patch.  This patch doesn't have any functional
change.

Reviewed-by: KAMEZAWA Hiroyuki <[email protected]>
Signed-off-by: KOSAKI Motohiro <[email protected]>
Acked-by: Rik van Riel <[email protected]>
Cc: Balbir Singh <[email protected]>
Cc: Daisuke Nishimura <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: KOSAKI Motohiro <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>