sctp: implement the receiver side for SACK-IMMEDIATELY extension

This patch implement the receiver side for SACK-IMMEDIATELY
extension:

  Section 4.2.  Receiver Side Considerations

  On reception of an SCTP packet containing a DATA chunk with the I-bit
  set, the receiver SHOULD NOT delay the sending of the corresponding
  SACK chunk and SHOULD send it back immediately.
Signed-off-by: NWei Yongjun <yjwei@cn.fujitsu.com>
Signed-off-by: NVlad Yasevich <vladislav.yasevich@hp.com>

net/sctp/sm_statefuns.c

@@ -2868,6 +2868,7 @@ sctp_disposition_t sctp_sf_eat_data_6_2(const struct sctp_endpoint *ep,
 					sctp_cmd_seq_t *commands)
 {
 	struct sctp_chunk *chunk = arg;
+	sctp_arg_t force = SCTP_NOFORCE();
 	int error;
 
 	if (!sctp_vtag_verify(chunk, asoc)) {
@@ -2901,6 +2902,9 @@ sctp_disposition_t sctp_sf_eat_data_6_2(const struct sctp_endpoint *ep,
 		BUG();
 	}
 
+	if (chunk->chunk_hdr->flags & SCTP_DATA_SACK_IMM)
+		force = SCTP_FORCE();
+
 	if (asoc->autoclose) {
 		sctp_add_cmd_sf(commands, SCTP_CMD_TIMER_RESTART,
 				SCTP_TO(SCTP_EVENT_TIMEOUT_AUTOCLOSE));
@@ -2929,7 +2933,7 @@ sctp_disposition_t sctp_sf_eat_data_6_2(const struct sctp_endpoint *ep,
 	 * more aggressive than the following algorithms allow.
 	 */
 	if (chunk->end_of_packet)
-		sctp_add_cmd_sf(commands, SCTP_CMD_GEN_SACK, SCTP_NOFORCE());
+		sctp_add_cmd_sf(commands, SCTP_CMD_GEN_SACK, force);
 
 	return SCTP_DISPOSITION_CONSUME;
 
@@ -2954,7 +2958,7 @@ sctp_disposition_t sctp_sf_eat_data_6_2(const struct sctp_endpoint *ep,
 
 discard_noforce:
 	if (chunk->end_of_packet)
-		sctp_add_cmd_sf(commands, SCTP_CMD_GEN_SACK, SCTP_NOFORCE());
+		sctp_add_cmd_sf(commands, SCTP_CMD_GEN_SACK, force);
 
 	return SCTP_DISPOSITION_DISCARD;
 consume: