net: ip6_tunnel: Unregister catch-all devices

commit 941ea91e87a6e879ed82dad4949f6234f2702bec upstream.

Similarly to the sit case, we need to remove the tunnels with no
addresses that have been moved to another network namespace.

Fixes: 0bd87628 ("ip6tnl: add x-netns support")
Signed-off-by: Hristo Venev <hristo@venev.name>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>