net: Teach vlans to cleanup as a pernet subsystem

Take advantage of the fact that an explicit rtnl_kill_links is
unnecessary (and skipping it improves batching), as network namespace
exit calls dellink on all remaining virtual devices, and
rtnl_link_unregister calls dellink on all outstanding devices in that
network namespace.  To do this we need to leave the vlan proc
directories in place until after network device exit time, which is
done by using register_pernet_subsys instead of
register_pernet_device.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>