[VLAN]: Introduce the vlan_net structure and init/exit net ops.

Unlike TUN, it is empty from the very beginning, and will
be eventually populated later.

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