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