[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

@@ -330,16 +330,6 @@ struct tcp_options {
 #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.