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>

net/ipv6/ip6_tunnel.c

@@ -1371,6 +1371,7 @@ static void ip6_tnl_dev_setup(struct net_device *dev)
 	dev->flags |= IFF_NOARP;
 	dev->addr_len = sizeof(struct in6_addr);
 	dev->features |= NETIF_F_NETNS_LOCAL;
+	dev->priv_flags &= ~IFF_XMIT_DST_RELEASE;
 }