macvlan: Propagate lowerdev MTU changes

When the physical MTU changes we should ensure that all existing MACVLAN
dev MTU do not exceed the new lowerdev MTU. This patch adds that
propagation.

Signed-off-by: Ding Tianhong <dingtianhong@huawei.com>
Reviewed-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/macvlan.c b/drivers/net/macvlan.c
index f0118d1..e03707d 100644
--- a/drivers/net/macvlan.c
+++ b/drivers/net/macvlan.c
@@ -1091,6 +1091,13 @@
 			netdev_update_features(vlan->dev);
 		}
 		break;
+	case NETDEV_CHANGEMTU:
+		list_for_each_entry(vlan, &port->vlans, list) {
+			if (vlan->dev->mtu <= dev->mtu)
+				continue;
+			dev_set_mtu(vlan->dev, dev->mtu);
+		}
+		break;
 	case NETDEV_UNREGISTER:
 		/* twiddle thumbs on netns device moves */
 		if (dev->reg_state != NETREG_UNREGISTERING)