sctp: send SHUTDOWN-ACK chunk back to the source.

SHUTDOWN-ACK is alaways sent to the primary path at the first time,
but should better transmit SHUTDOWN-ACK chunk to the same destination
transport address from which it received the SHUTDOWN chunk.
Based on the work from Wei Yongjun <yjwei@cn.fujitsu.com>.
Signed-off-by: NVlad Yasevich <vladislav.yasevich@hp.com>

net/sctp/sm_sideeffect.c

@@ -697,11 +697,15 @@ static void sctp_cmd_setup_t2(sctp_cmd_seq_t *cmds,
 {
 	struct sctp_transport *t;
 
-	t = sctp_assoc_choose_alter_transport(asoc,
+	if (chunk->transport)
+		t = chunk->transport;
+	else {
+		t = sctp_assoc_choose_alter_transport(asoc,
 					      asoc->shutdown_last_sent_to);
+		chunk->transport = t;
+	}
 	asoc->shutdown_last_sent_to = t;
 	asoc->timeouts[SCTP_EVENT_TIMEOUT_T2_SHUTDOWN] = t->rto;
-	chunk->transport = t;
 }
 
 /* Helper function to change the state of an association. */