[VLAN]: Allow vlan devices registration in net namespaces.

This one is similar to what I've done for TUN - set the proper
net after device allocation and clean VLANs on net exit (use the
rtnl_kill_links helper finally).

Plus, drop explicit init_net usage and net != &init_net checks.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Acked-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>