sctp: fix the issue sctp requeue auth chunk incorrectly

sctp needs to queue auth chunk back when we know that we are going
to generate another segment. But commit f1533cce60d1 ("sctp: fix
panic when sending auth chunks") requeues the last chunk processed
which is probably not the auth chunk.

It causes panic when calculating the MAC in sctp_auth_calculate_hmac(),
as the incorrect offset of the auth chunk in skb->data.

This fix is to requeue it by using packet->auth.

Fixes: f1533cce60d1 ("sctp: fix panic when sending auth chunks")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sctp/output.c

@@ -610,7 +610,8 @@ int sctp_packet_transmit(struct sctp_packet *packet, gfp_t gfp)
 				/* We will generate more packets, so re-queue
 				 * auth chunk.
 				 */
-				list_add(&chunk->list, &packet->chunk_list);
+				list_add(&packet->auth->list,
+					 &packet->chunk_list);
 			} else {
 				sctp_chunk_free(packet->auth);
 				packet->auth = NULL;