tcp rack: improve handling of front states

When the RACK stack wants to send a FIN, but still has outstanding
or unsent data, it sends a challenge ack. Don't do this when the
TCP endpoint is still in the front states, since it does not
make sense.
Reviewed by: rrs
MFC after: 3 days
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D45122