sctp: implement assign_number for sctp_stream_interleave

assign_number is added as a member of sctp_stream_interleave, used
to assign ssn for data or mid (message id) for idata, called in
sctp_packet_append_data. sctp_chunk_assign_ssn is left as it is,
and sctp_chunk_assign_mid is added for sctp_stream_interleave_1.

This procedure is described in section 2.2.2 of RFC8260.

All sizeof(struct sctp_data_chunk) in tx path is replaced with
sctp_datachk_len, to make it right for idata as well. And also
adjust sctp_chunk_is_data for SCTP_CID_I_DATA.

After this patch, idata can be built and sent in tx path.

Note that if sp strm_interleave is set, it has to wait_connect in
sctp_sendmsg, as asoc intl_enable need to be known after 4 shake-
hands, to decide if it should use data or idata later. data and
idata can't be mixed to send in one asoc.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sctp/ulpevent.c

@@ -443,8 +443,8 @@ struct sctp_ulpevent *sctp_ulpevent_make_send_failed(
 		goto fail;
 
 	/* Pull off the common chunk header and DATA header.  */
-	skb_pull(skb, sizeof(struct sctp_data_chunk));
-	len -= sizeof(struct sctp_data_chunk);
+	skb_pull(skb, sctp_datachk_len(&asoc->stream));
+	len -= sctp_datachk_len(&asoc->stream);
 
 	/* Embed the event fields inside the cloned skb.  */
 	event = sctp_skb2event(skb);