vmscan: avoid setting zone congested if no page dirty

nr_dirty and nr_congested are increased only when the page is dirty.  So
if all pages are clean, both them will be zero.  In this case, we should
not mark the zone congested.
Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Reviewed-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Acked-by: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmscan.c

@@ -913,7 +913,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 	 * back off and wait for congestion to clear because further reclaim
 	 * will encounter the same problem
 	 */
-	if (nr_dirty == nr_congested)
+	if (nr_dirty == nr_congested && nr_dirty != 0)
 		zone_set_flag(zone, ZONE_CONGESTED);
 
 	free_page_list(&free_pages);