slub: improve kmem_cache_destroy() error message

As pointed out by Ingo, the SLUB warning of calling kmem_cache_destroy()
with cache that still has objects triggers in practice. So turn this
WARN_ON() into a nice SLUB specific error message to avoid people
confusing it to a SLUB bug.

Cc: Ingo Molnar <mingo@elte.hu>
Acked-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>

mm/slub.c

@@ -2426,8 +2426,11 @@ void kmem_cache_destroy(struct kmem_cache *s)
 	if (!s->refcount) {
 		list_del(&s->list);
 		up_write(&slub_lock);
-		if (kmem_cache_close(s))
-			WARN_ON(1);
+		if (kmem_cache_close(s)) {
+			printk(KERN_ERR "SLUB %s: %s called for cache that "
+				"still has objects.\n", s->name, __func__);
+			dump_stack();
+		}
 		sysfs_slab_remove(s);
 	} else
 		up_write(&slub_lock);