When attempting to transmit a packet, if the system fails to allocate

a mbuf, it may return without setting any timers.  If no more data is
scheduled to be transmitted (this was a FIN) the system will sit in
LAST_ACK state forever.

Thus, when mbuf allocation fails, set the retransmit timer if neither
the retransmit or persist timer is already pending.

Problem discovered by:  Mike Silbersack (silby@silby.com)
Pushed for a fix by:    Bosko Milekic <bmilekic@dsuper.net>
Reviewed by:            jayanth

sys/netinet/tcp_output.c

@@ -859,6 +859,10 @@ tcp_output(tp)
 	if (error) {
 out:
 		if (error == ENOBUFS) {
+	                if (!callout_active(tp->tt_rexmt) &&
+                            !callout_active(tp->tt_persist))
+	                        callout_reset(tp->tt_rexmt, tp->t_rxtcur,
+                                      tcp_timer_rexmt, tp);
 			tcp_quench(tp->t_inpcb, 0);
 			return (0);
 		}