mm: fix testorder interaction between two kswapd patches

Adjusting cc715d9 "mm: vmscan: forcibly scan highmem if there are
too many buffer_heads pinning highmem" for -stable reveals that it was
slightly wrong once on top of fe2c2a1 "vmscan: reclaim at order 0
when compaction is enabled", which specifically adds testorder for the
zone_watermark_ok_safe() test.

Signed-off-by: Hugh Dickins <[email protected]>
Acked-by: Mel Gorman <[email protected]>
Acked-by: Rik van Riel <[email protected]>
Cc: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/vmscan.c

@@ -2817,7 +2817,7 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
 				testorder = 0;
 
 			if ((buffer_heads_over_limit && is_highmem_idx(i)) ||
-				    !zone_watermark_ok_safe(zone, order,
+				    !zone_watermark_ok_safe(zone, testorder,
 					high_wmark_pages(zone) + balance_gap,
 					end_zone, 0)) {
 				shrink_zone(priority, zone, &sc);