rtnl: fix error path when adding an iface with a link net

If an error occurs when the netdevice is moved to the link netns, a full cleanup
must be done.

Fixes: 317f4810e45e ("rtnl: allow to create device with IFLA_LINK_NETNSID set")
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/rtnetlink.c

@@ -2172,8 +2172,11 @@ replay:
 			goto out;
 		}
 
-		if (link_net)
+		if (link_net) {
 			err = dev_change_net_namespace(dev, dest_net, ifname);
+			if (err < 0)
+				unregister_netdevice(dev);
+		}
 out:
 		if (link_net)
 			put_net(link_net);