mm: introduce for_each_populated_zone() macro

Impact: cleanup

In almost cases, for_each_zone() is used with populated_zone().  It's
because almost function doesn't need memoryless node information.
Therefore, for_each_populated_zone() can help to make code simplify.

This patch has no functional change.

[akpm@linux-foundation.org: small cleanup]
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Reviewed-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/kernel/power/swsusp.c b/kernel/power/swsusp.c
index a92c914..1ee66364 100644
--- a/kernel/power/swsusp.c
+++ b/kernel/power/swsusp.c
@@ -229,17 +229,16 @@
 		size = count_data_pages() + PAGES_FOR_IO + SPARE_PAGES;
 		tmp = size;
 		size += highmem_size;
-		for_each_zone (zone)
-			if (populated_zone(zone)) {
-				tmp += snapshot_additional_pages(zone);
-				if (is_highmem(zone)) {
-					highmem_size -=
+		for_each_populated_zone(zone) {
+			tmp += snapshot_additional_pages(zone);
+			if (is_highmem(zone)) {
+				highmem_size -=
 					zone_page_state(zone, NR_FREE_PAGES);
-				} else {
-					tmp -= zone_page_state(zone, NR_FREE_PAGES);
-					tmp += zone->lowmem_reserve[ZONE_NORMAL];
-				}
+			} else {
+				tmp -= zone_page_state(zone, NR_FREE_PAGES);
+				tmp += zone->lowmem_reserve[ZONE_NORMAL];
 			}
+		}
 
 		if (highmem_size < 0)
 			highmem_size = 0;