wireguard: implement header_ops->parse_protocol for AF_PACKET

WireGuard uses skb->protocol to determine packet type, and bails out if
it's not set or set to something it's not expecting. 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 wireguard then
rejects the skb. So, this wires up the ip_tunnel handler for layer 3
packets for that case.
Reported-by: Hans Wippel <ndev@hwipl.net>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/wireguard/device.c

@@ -262,6 +262,7 @@ static void wg_setup(struct net_device *dev)
 			     max(sizeof(struct ipv6hdr), sizeof(struct iphdr));
 
 	dev->netdev_ops = &netdev_ops;
+	dev->header_ops = &ip_tunnel_header_ops;
 	dev->hard_header_len = 0;
 	dev->addr_len = 0;
 	dev->needed_headroom = DATA_PACKET_HEAD_ROOM;