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: 317f4810

 ("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>