mm: page_alloc: do not treat a zone that cannot be used for dirty pages as "full"

If a zone cannot be used for a dirty page then it gets marked "full" which
is cached in the zlc and later potentially skipped by allocation requests
that have nothing to do with dirty zones.

Signed-off-by: Mel Gorman <mgorman@suse.de>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -1967,7 +1967,7 @@ zonelist_scan:
 		 */
 		if ((alloc_flags & ALLOC_WMARK_LOW) &&
 		    (gfp_mask & __GFP_WRITE) && !zone_dirty_ok(zone))
-			goto this_zone_full;
+			continue;
 
 		mark = zone->watermark[alloc_flags & ALLOC_WMARK_MASK];
 		if (!zone_watermark_ok(zone, order, mark,