[kobject] Add warning + back trace if kobject_get() is called with 0 refcount.

lib/kobject.c

@@ -347,9 +347,10 @@ struct kobject * kobject_get(struct kobject * kobj)
 {
 	struct kobject * ret = kobj;
 
-	if (kobj)
+	if (kobj) {
+		WARN_ON(!atomic_read(&kobj->refcount));
 		atomic_inc(&kobj->refcount);
-	else
+	} else
 		ret = NULL;
 	return ret;
 }