vmscan: count pages into balanced for zone with good watermark

It's possible a zone watermark is ok when entering the balance_pgdat()
loop, while the zone is within the requested classzone_idx.  Count pages
from this zone into `balanced'.  In this way, we can skip shrinking zones
too much for high order allocation.
Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Acked-by: Mel Gorman <mgorman@suse.de>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmscan.c

@@ -2748,6 +2748,8 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
 
 			/* If balanced, clear the congested flag */
 			zone_clear_flag(zone, ZONE_CONGESTED);
+			if (i <= *classzone_idx)
+				balanced += zone->present_pages;
 		}
 	}