Fix TCP Fast Open for the TCP RACK stack.

• Fix a bug where the SYN handling during established state was applied to a front state.
• Move a check for retransmission after the timer handling. This was suppressing timer based retransmissions.
• Fix an off-by one byte in the sequence number of retransmissions.
• Apply fixes corresponding to https://svnweb.freebsd.org/changeset/base/336934

Reviewed by: rrs@
Approved by: re (kib@)
MFC after: 1 month
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D16912