vmscan: kswapd: don't retry balance_pgdat() if all zones are unreclaimable

Commit f50de2d3 (vmscan: have kswapd sleep for a short interval and double
check it should be asleep) can cause kswapd to enter an infinite loop if
running on a single-CPU system. If all zones are unreclaimble,
sleeping_prematurely return 1 and kswapd will call balance_pgdat() again.
but it's totally meaningless, balance_pgdat() doesn't anything against
unreclaimable zone!

Signed-off-by: KOSAKI Motohiro <[e-mail]>
Cc: Mel Gorman <[e-mail]>
Reported-by: Will Newton <[e-mail]>
Reviewed-by: Minchan Kim <[e-mail]>
Reviewed-by: Rik van Riel <[e-mail]>
Tested-by: Will Newton <[e-mail]>
Reviewed-by: Wu Fengguang <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/vmscan.c

···
 		if (!populated_zone(zone))
 			continue;
 
+		if (zone_is_all_unreclaimable(zone))
+			continue;
+
 		if (!zone_watermark_ok(zone, order, high_wmark_pages(zone),
 								0, 0))
 			return 1;