mm/page_alloc.c: fix wrong initialization when sysctl_min_unmapped_ratio changes

Before resetting min_unmapped_pages, we need to initialize
min_unmapped_pages rather than min_slab_pages.

Fixes: a5f5f91da6 (mm: convert zone_reclaim to node_reclaim)
Link: http://lkml.kernel.org/r/1470724248-26780-1-git-send-email-iamjoonsoo.kim@lge.com
Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -6854,7 +6854,7 @@ int sysctl_min_unmapped_ratio_sysctl_handler(struct ctl_table *table, int write,
 		return rc;
 
 	for_each_online_pgdat(pgdat)
-		pgdat->min_slab_pages = 0;
+		pgdat->min_unmapped_pages = 0;
 
 	for_each_zone(zone)
 		zone->zone_pgdat->min_unmapped_pages += (zone->managed_pages *