Use appropriate MSS value when populating the TCP FO client cookie cache

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 RFC 7413 is used.

Reviewed by: kbowling@
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D16175