memory hotplug: fix unnecessary calling of init_currenty_empty_zone()

zone->present_pages is updated in online_pages().  But, __add_zone() can be
called twice or more before calling online_pages().  So,
init_currenty_empty_zone() can be called unnecessary times.  It is cause of
memory leak of zone's wait_table.
Signed-off-by: Yasunori Goto <y-goto@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memory_hotplug.c

@@ -65,7 +65,7 @@ static int __add_zone(struct zone *zone, unsigned long phys_start_pfn)
 	int zone_type;
 
 	zone_type = zone - pgdat->node_zones;
-	if (!populated_zone(zone)) {
+	if (!zone->wait_table) {
 		int ret = 0;
 		ret = init_currently_empty_zone(zone, phys_start_pfn,
 						nr_pages, MEMMAP_HOTPLUG);