mm/page_alloc: fix the range check for backward merging

The current check for 'backward merging' within add_active_range() does
not seem correct.  start_pfn must be compared against
early_node_map[i].start_pfn (and NOT against .end_pfn) to find out whether
the new region is backward-mergeable with the existing range.
Signed-off-by: Kazuhisa Ichikawa <ki@epsilou.com>
Acked-by: David Rientjes <rientjes@google.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Christoph Lameter <cl@linux-foundation.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -3998,7 +3998,7 @@ void __init add_active_range(unsigned int nid, unsigned long start_pfn,
 		}
 
 		/* Merge backward if suitable */
-		if (start_pfn < early_node_map[i].end_pfn &&
+		if (start_pfn < early_node_map[i].start_pfn &&
 				end_pfn >= early_node_map[i].start_pfn) {
 			early_node_map[i].start_pfn = start_pfn;
 			return;