[tcp] Fix ECN on finalizing sessions.

A subtle oversight would subtly change new data packets
sent after a shutdown() or close() call, while the send
buffer is still draining.

MFC after: 3 days
Reviewed By: #transport, tuexen
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D29616

sys/netinet/tcp_output.c

@@ -1181,7 +1181,7 @@ send:
 			tp->t_flags2 &= ~TF2_ECN_SND_ECE;
 	}
 
-	if (tp->t_state == TCPS_ESTABLISHED &&
+	if (TCPS_HAVEESTABLISHED(tp->t_state) &&
 	    (tp->t_flags2 & TF2_ECN_PERMIT)) {
 		/*
 		 * If the peer has ECN, mark data packets with