[tcpip] Provide tcpip_mtu() to determine the maximum transmission unit

Provide the function tcpip_mtu() to allow external code to determine
the (transport-layer) maximum transmission unit for a given socket
address.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/ipv6.c

@@ -988,6 +988,7 @@ struct net_protocol ipv6_protocol __net_protocol = {
 struct tcpip_net_protocol ipv6_tcpip_protocol __tcpip_net_protocol = {
 	.name = "IPv6",
 	.sa_family = AF_INET6,
+	.header_len = sizeof ( struct ipv6_header ),
 	.tx = ipv6_tx,
 	.netdev = ipv6_netdev,
 };