[SIT]: Make the fallback tunnel device per-net

Allocate and register one in sit_init_net, use sitn->fb_tunnel_dev
over the code and unregister one in sit_exit_net.

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