team: fix mtu setting

Now it is not possible to set mtu to team device which has a port
enslaved to it. The reason is that when team_change_mtu() calls
dev_set_mtu() for port device, notificator for NETDEV_PRECHANGEMTU
event is called and team_device_event() returns NOTIFY_BAD forbidding
the change. So fix this by returning NOTIFY_DONE here in case team is
changing mtu in team_change_mtu().

Introduced-by: 3d249d4c "net: introduce ethernet teaming device"
Signed-off-by: Jiri Pirko <[e-mail]>
Acked-by: Flavio Leitner <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/team/team.c

···
 	 * to traverse list in reverse under rcu_read_lock
 	 */
 	mutex_lock(&team->lock);
+	team->port_mtu_change_allowed = true;
 	list_for_each_entry(port, &team->port_list, list) {
 		err = dev_set_mtu(port->dev, new_mtu);
 		if (err) {
···
 			goto unwind;
 		}
 	}
+	team->port_mtu_change_allowed = false;
 	mutex_unlock(&team->lock);
 
 	dev->mtu = new_mtu;
···
 unwind:
 	list_for_each_entry_continue_reverse(port, &team->port_list, list)
 		dev_set_mtu(port->dev, dev->mtu);
+	team->port_mtu_change_allowed = false;
 	mutex_unlock(&team->lock);
 
 	return err;
···
 		break;
 	case NETDEV_PRECHANGEMTU:
 		/* Forbid to change mtu of underlaying device */
-		return NOTIFY_BAD;
+		if (!port->team->port_mtu_change_allowed)
+			return NOTIFY_BAD;
+		break;
 	case NETDEV_PRE_TYPE_CHANGE:
 		/* Forbid to change type of underlaying device */
 		return NOTIFY_BAD;

include/linux/if_team.h

···
 	bool user_carrier_enabled;
 	bool queue_override_enabled;
 	struct list_head *qom_lists; /* array of queue override mapping lists */
+	bool port_mtu_change_allowed;
 	struct {
 		unsigned int count;
 		unsigned int interval; /* in ms */