sit: fix regression: do not release skb->dst before xmit

The sit module makes use of skb->dst in it's xmit function, so since
93f154b5 ("net: release dst entry in dev_hard_start_xmit()") sit
tunnels are broken, because the flag IFF_XMIT_DST_RELEASE is not
unset.

This patch unsets that flag for sit devices to fix this
regression.
Signed-off-by: Sascha Hlusiak <contact@saschahlusiak.de>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/sit.c

@@ -1018,6 +1018,7 @@ static void ipip6_tunnel_setup(struct net_device *dev)
 	dev->hard_header_len 	= LL_MAX_HEADER + sizeof(struct iphdr);
 	dev->mtu		= ETH_DATA_LEN - sizeof(struct iphdr);
 	dev->flags		= IFF_NOARP;
+	dev->priv_flags	       &= ~IFF_XMIT_DST_RELEASE;
 	dev->iflink		= 0;
 	dev->addr_len		= 4;
 	dev->features		|= NETIF_F_NETNS_LOCAL;