[TUN]: Introduce the tun_net structure and init/exit net ops.

This is the first step in making tuntap devices work in net 
namespaces. The structure mentioned is pointed by generic
net pointer with tun_net_id id, and tun driver fills one on 
its load. It will contain only the tun devices list.

So declare this structure and introduce net init and exit hooks.

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