Use appropriate MSS when populating the TCP fast open client cookie cache

[tuexen]

Summary

When a client receives a SYN-ACK segment with a TFP fast open cookie, but without an MSS option, an MSS value from uninitialised stack memory is used. This patch ensures that in case no MSS option is included in the SYN-ACK, the appropriate value as given in the TCP fast open RFC is used.

Test Plan

Use a packetdrill script to trigger the condition and use D14554 to verify the correct MSS is used.