macvlan: Fix one possible double free

Because the macvlan_uninit would free the macvlan port, so there is one
double free case in macvlan_common_newlink. When the macvlan port is just
created, then register_netdevice or netdev_upper_dev_link failed and they
would invoke macvlan_uninit. Then it would reach the macvlan_port_destroy
which triggers the double free.
Signed-off-by: Gao Feng <gfree.wind@vip.163.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/macvlan.c

@@ -1444,9 +1444,14 @@ int macvlan_common_newlink(struct net *src_net, struct net_device *dev,
 	return 0;
 
 unregister_netdev:
+	/* macvlan_uninit would free the macvlan port */
 	unregister_netdevice(dev);
+	return err;
 destroy_macvlan_port:
-	if (create)
+	/* the macvlan port may be freed by macvlan_uninit when fail to register.
+	 * so we destroy the macvlan port only when it's valid.
+	 */
+	if (create && macvlan_port_get_rtnl(dev))
 		macvlan_port_destroy(port->dev);
 	return err;
 }