Don't set FIN on a retransmitted segment after a FIN has been sent,

unless the segment really contains the last of the data for the stream.

PR:		kern/34619
Obtained from:	OpenBSD (tcp_output.c rev 1.47)
Noticed by:	Joseph Ishac
Reviewed by:	George Neville-Neil

sys/netinet/tcp_output.c

@@ -292,7 +292,7 @@ again:
 		len = tp->t_maxseg;
 		sendalot = 1;
 	}
-	if (SEQ_LT(tp->snd_nxt + len, tp->snd_una + so->so_snd.sb_cc))
+	if (off + len < so->so_snd.sb_cc)
 		flags &= ~TH_FIN;
 
 	recwin = sbspace(&so->so_rcv);