ip6_gre: ip6gre_tap device should keep dst

The patch 'ip_gre: ipgre_tap device should keep dst' fixed
a issue that ipgre_tap mtu couldn't be updated in tx path.

The same fix is needed for ip6gre_tap as well.
Signed-off-by: NXin Long <lucien.xin@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/ip6_gre.c

@@ -1311,6 +1311,7 @@ static void ip6gre_tap_setup(struct net_device *dev)
 	dev->features |= NETIF_F_NETNS_LOCAL;
 	dev->priv_flags &= ~IFF_TX_SKB_SHARING;
 	dev->priv_flags |= IFF_LIVE_ADDR_CHANGE;
+	netif_keep_dst(dev);
 }
 
 static bool ip6gre_netlink_encap_parms(struct nlattr *data[],