mm/mmap: use hotplug_memory_notifier() directly

Since patch 3163462 has already updated the minimum gcc version to 5.1.
The previous problem mentioned in patch f02c696 is not existed. So we
can now revert to use hotplug_memory_notifier() directly rather than
register_hotmemory_notifier().

Signed-off-by: Liu Shixin <liushixin2@huawei.com>

mm/mmap.c

@@ -3744,13 +3744,9 @@ static int reserve_mem_notifier(struct notifier_block *nb,
 	return NOTIFY_OK;
 }
 
-static struct notifier_block reserve_mem_nb = {
-	.notifier_call = reserve_mem_notifier,
-};
-
 static int __meminit init_reserve_notifier(void)
 {
-	if (register_hotmemory_notifier(&reserve_mem_nb))
+	if (hotplug_memory_notifier(&reserve_mem_notifier, 0))
 		pr_err("Failed registering memory add/remove notifier for admin reserve\n");
 
 	return 0;