[PATCH] fix kswapd throttling

kswapd currently takes a throttling nap even if it freed all the pages it
was asked to free.

Change it so we only throttle if reclaim is not being sufficiently
successful.

mm/vmscan.c

@@ -930,7 +930,8 @@ static int balance_pgdat(pg_data_t *pgdat, int nr_pages, struct page_state *ps)
 		}
 		if (all_zones_ok)
 			break;
-		blk_congestion_wait(WRITE, HZ/10);
+		if (to_free > 0)
+			blk_congestion_wait(WRITE, HZ/10);
 	}
 	return nr_pages - to_free;
 }