sit: strictly restrict incoming traffic to tunnel link device

Check link device when looking up a tunnel. When a tunnel is
linked to a interface, traffic from a different interface must not
reach the tunnel.

This also allows creating of multiple tunnels with the same
endpoints, if the link device differs.

Signed-off-by: Sascha Hlusiak <contact@saschahlusiak.de>
Signed-off-by: David S. Miller <davem@davemloft.net>