net: sit: implement header_ops->parse_protocol for AF_PACKET

Sit uses skb->protocol to determine packet type, and bails out if it's
not set. For AF_PACKET injection, we need to support its call chain of:

    packet_sendmsg -> packet_snd -> packet_parse_headers ->
      dev_parse_header_protocol -> parse_protocol

Without a valid parse_protocol, this returns zero, and sit rejects the
skb. So, this wires up the ip_tunnel handler for layer 3 packets for
that case.
Reported-by: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/sit.c

@@ -1421,6 +1421,7 @@ static void ipip6_tunnel_setup(struct net_device *dev)
 	int t_hlen = tunnel->hlen + sizeof(struct iphdr);
 
 	dev->netdev_ops		= &ipip6_netdev_ops;
+	dev->header_ops		= &ip_tunnel_header_ops;
 	dev->needs_free_netdev	= true;
 	dev->priv_destructor	= ipip6_dev_free;