[tcp] Calculate correct MSS from peer address

iPXE currently advertises a fixed MSS of 1460, which is correct only
for IPv4 over Ethernet.  For IPv6 over Ethernet, the value should be
1440 (allowing for the larger IPv6 header).  For non-Ethernet link
layers, the value should reflect the MTU of the underlying network
device.

Use tcpip_mtu() to calculate the transport-layer MTU associated with
the peer address, and calculate the MSS to allow for an optionless TCP
header as per RFC 6691.

As a side benefit, we can now fail a connection immediately with a
meaningful error message if we have no route to the destination
address.

Reported-by: Anton D. Kachalov <mouse@yandex-team.ru>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/tcp.h

 #define TCP_PATH_MTU							\
 	( 1280 - 40 /* IPv6 */ - 20 /* TCP */ - 12 /* TCP timestamp */ )
 
-/**
- * Advertised TCP MSS
- *
- * We currently hardcode this to a reasonable value and hope that the
- * sender uses path MTU discovery.  The alternative is breaking the
- * abstraction layer so that we can find out the MTU from the IP layer
- * (which would have to find out from the net device layer).
- */
-#define TCP_MSS 1460
-
 /** TCP maximum segment lifetime
  *
  * Currently set to 2 minutes, as per RFC 793.

src/net/tcp.c

 	struct sockaddr_tcpip peer;
 	/** Local port */
 	unsigned int local_port;
+	/** Maximum segment size */
+	size_t mss;
 
 	/** Current TCP state */
 	unsigned int tcp_state;
 	struct sockaddr_tcpip *st_peer = ( struct sockaddr_tcpip * ) peer;
 	struct sockaddr_tcpip *st_local = ( struct sockaddr_tcpip * ) local;
 	struct tcp_connection *tcp;
+	size_t mtu;
 	int port;
 	int rc;
 
 	INIT_LIST_HEAD ( &tcp->rx_queue );
 	memcpy ( &tcp->peer, st_peer, sizeof ( tcp->peer ) );
 
+	/* Calculate MSS */
+	mtu = tcpip_mtu ( &tcp->peer );
+	if ( ! mtu ) {
+		DBGC ( tcp, "TCP %p has no route to %s\n",
+		       tcp, sock_ntoa ( peer ) );
+		rc = -ENETUNREACH;
+		goto err;
+	}
+	tcp->mss = ( mtu - sizeof ( struct tcp_header ) );
+
 	/* Bind to local port */
 	port = tcpip_bind ( st_local, tcp_port_available );
 	if ( port < 0 ) {
 		mssopt = iob_push ( iobuf, sizeof ( *mssopt ) );
 		mssopt->kind = TCP_OPTION_MSS;
 		mssopt->length = sizeof ( *mssopt );
-		mssopt->mss = htons ( TCP_MSS );
+		mssopt->mss = htons ( tcp->mss );
 		wsopt = iob_push ( iobuf, sizeof ( *wsopt ) );
 		wsopt->nop = TCP_OPTION_NOP;
 		wsopt->wsopt.kind = TCP_OPTION_WS;