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/snapshot.c b/kernel/power/snapshot.c
index f5fc2d7..33e2e4a 100644
--- a/kernel/power/snapshot.c
+++ b/kernel/power/snapshot.c
@@ -321,13 +321,10 @@
 
 	INIT_LIST_HEAD(list);
 
-	for_each_zone(zone) {
+	for_each_populated_zone(zone) {
 		unsigned long zone_start, zone_end;
 		struct mem_extent *ext, *cur, *aux;
 
-		if (!populated_zone(zone))
-			continue;
-
 		zone_start = zone->zone_start_pfn;
 		zone_end = zone->zone_start_pfn + zone->spanned_pages;
 
@@ -804,8 +801,8 @@
 	struct zone *zone;
 	unsigned int cnt = 0;
 
-	for_each_zone(zone)
-		if (populated_zone(zone) && is_highmem(zone))
+	for_each_populated_zone(zone)
+		if (is_highmem(zone))
 			cnt += zone_page_state(zone, NR_FREE_PAGES);
 
 	return cnt;