net: ipip: implement header_ops->parse_protocol for AF_PACKET

Ipip 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 ipip rejects the
skb. So, this wires up the ip_tunnel handler for layer 3 packets for
that case.
Signed-off-by: NJason A. Donenfeld <Jason@zx2c4.com>
Acked-by: NWillem de Bruijn <willemb@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/ipip.c

@@ -361,6 +361,7 @@ static const struct net_device_ops ipip_netdev_ops = {
 static void ipip_tunnel_setup(struct net_device *dev)
 {
 	dev->netdev_ops		= &ipip_netdev_ops;
+	dev->header_ops		= &ip_tunnel_header_ops;
 
 	dev->type		= ARPHRD_TUNNEL;
 	dev->flags		= IFF_NOARP;

net/ipv6/ip6_tunnel.c

@@ -1846,6 +1846,7 @@ static const struct net_device_ops ip6_tnl_netdev_ops = {
 static void ip6_tnl_dev_setup(struct net_device *dev)
 {
 	dev->netdev_ops = &ip6_tnl_netdev_ops;
+	dev->header_ops = &ip_tunnel_header_ops;
 	dev->needs_free_netdev = true;
 	dev->priv_destructor = ip6_dev_free;