slqb: correctly return value for notification handler

Correctly return value for notification handler. The bug causes other
handlers are ignored and panic kernel.
Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>

mm/slqb.c

@@ -2846,7 +2846,10 @@ static int slab_memory_callback(struct notifier_block *self,
 		break;
 	}
 
+	if (ret)
 		ret = notifier_from_errno(ret);
+	else
+		ret = NOTIFY_OK;
 	return ret;
 }