mm/slub.c: use register_hotmemory_notifier()

Squishes a statement-with-no-effect warning, removes some ifdefs and
shrinks .text by 2 bytes.

Note that this code fails to check for blocking_notifier_chain_register()
failures.

Cc: Pekka Enberg <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/slub.c

···
 #include <linux/slab.h>
 #include "slab.h"
 #include <linux/proc_fs.h>
+#include <linux/notifier.h>
 #include <linux/seq_file.h>
 #include <linux/kmemcheck.h>
 #include <linux/cpu.h>
···
 }
 EXPORT_SYMBOL(kmem_cache_shrink);
 
-#if defined(CONFIG_MEMORY_HOTPLUG)
 static int slab_mem_going_offline_callback(void *arg)
 {
 	struct kmem_cache *s;
···
 	return ret;
 }
 
-#endif /* CONFIG_MEMORY_HOTPLUG */
+static struct notifier_block slab_memory_callback_nb = {
+	.notifier_call = slab_memory_callback,
+	.priority = SLAB_CALLBACK_PRI,
+};
 
 /********************************************************************
  *			Basic setup of slabs
···
 	create_boot_cache(kmem_cache_node, "kmem_cache_node",
 		sizeof(struct kmem_cache_node), SLAB_HWCACHE_ALIGN);
 
-	hotplug_memory_notifier(slab_memory_callback, SLAB_CALLBACK_PRI);
+	register_hotmemory_notifier(&slab_memory_callback_nb);
 
 	/* Able to allocate the per node structures */
 	slab_state = PARTIAL;