ip6_tunnel dont update the mtu on the route.

The ip6_tunnel device did not unset the flag,
IFF_XMIT_DST_RELEASE. This will make the dev layer
to release the dst before calling the tunnel.
The tunnel will not update any mtu/pmtu info, since
it does not have a dst on the skb.
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>

Signed-off-by: David S. Miller <davem@davemloft.net>