mm/slub.c: 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/slub.c

@@ -4772,11 +4772,6 @@ static int slab_memory_callback(struct notifier_block *self,
 	return ret;
 }
 
-static struct notifier_block slab_memory_callback_nb = {
-	.notifier_call = slab_memory_callback,
-	.priority = SLAB_CALLBACK_PRI,
-};
-
 /********************************************************************
  *			Basic setup of slabs
  *******************************************************************/
@@ -4842,7 +4837,7 @@ void __init kmem_cache_init(void)
 	create_boot_cache(kmem_cache_node, "kmem_cache_node",
 		sizeof(struct kmem_cache_node), SLAB_HWCACHE_ALIGN, 0, 0);
 
-	register_hotmemory_notifier(&slab_memory_callback_nb);
+	hotplug_memory_notifier(slab_memory_callback, SLAB_CALLBACK_PRI);
 
 	/* Able to allocate the per node structures */
 	slab_state = PARTIAL;