memory_hotplug: use pgdat_resize_lock() in __offline_pages()

mmzone.h documents node_size_lock (which pgdat_resize_lock() locks) as
follows:

        * Must be held any time you expect node_start_pfn, node_present_pages
        * or node_spanned_pages stay constant.  [...]

So actually hold it when we update node_present_pages in __offline_pages().

[akpm@linux-foundation.org: fix build]
Signed-off-by: Cody P Schafer <cody@linux.vnet.ibm.com>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memory_hotplug.c

@@ -1492,6 +1492,7 @@ static int __ref __offline_pages(unsigned long start_pfn,
 	unsigned long pfn, nr_pages, expire;
 	long offlined_pages;
 	int ret, drain, retry_max, node;
+	unsigned long flags;
 	struct zone *zone;
 	struct memory_notify arg;
 
@@ -1585,7 +1586,11 @@ repeat:
 	/* removal success */
 	zone->managed_pages -= offlined_pages;
 	zone->present_pages -= offlined_pages;
+
+	pgdat_resize_lock(zone->zone_pgdat, &flags);
 	zone->zone_pgdat->node_present_pages -= offlined_pages;
+	pgdat_resize_unlock(zone->zone_pgdat, &flags);
+
 	totalram_pages -= offlined_pages;
 
 	init_per_zone_wmark_min();