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

sys/netinet/tcp_stacks/rack.c

@@ -22776,7 +22776,8 @@ rack_output(struct tcpcb *tp)
 		 * is acked first.
 		 */
 		flags &= ~TH_FIN;
-		if ((sbused(sb) == (tp->snd_max - tp->snd_una)) &&
+		if (TCPS_HAVEESTABLISHED(tp->t_state) &&
+		    (sbused(sb) == (tp->snd_max - tp->snd_una)) &&
 		    ((tp->snd_max - tp->snd_una) <= segsiz)) {
 			/*
 			 * Ok less than or right at a MSS is