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/mm/vmscan.c b/mm/vmscan.c
index 1bca60f..301f057 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2061,11 +2061,9 @@
 	struct zone *zone;
 	unsigned long ret = 0;
 
-	for_each_zone(zone) {
+	for_each_populated_zone(zone) {
 		enum lru_list l;
 
-		if (!populated_zone(zone))
-			continue;
 		if (zone_is_all_unreclaimable(zone) && prio != DEF_PRIORITY)
 			continue;