sctp: implement the sender side for SACK-IMMEDIATELY extension

This patch implement the sender side for SACK-IMMEDIATELY
extension.

  Section 4.1.  Sender Side Considerations

  Whenever the sender of a DATA chunk can benefit from the
  corresponding SACK chunk being sent back without delay, the sender
  MAY set the I-bit in the DATA chunk header.

  Reasons for setting the I-bit include

  o  The sender is in the SHUTDOWN-PENDING state.

  o  The application requests to set the I-bit of the last DATA chunk
     of a user message when providing the user message to the SCTP
     implementation.

Signed-off-by: Wei Yongjun <yjwei@cn.fujitsu.com>
Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>

include/net/sctp/user.h

@@ -206,6 +206,7 @@ enum sctp_sinfo_flags {
 	SCTP_UNORDERED = 1,  /* Send/receive message unordered. */
 	SCTP_ADDR_OVER = 2,  /* Override the primary destination. */
 	SCTP_ABORT=4,        /* Send an ABORT message to the peer. */
+	SCTP_SACK_IMMEDIATELY = 8,	/* SACK should be sent without delay */
 	SCTP_EOF=MSG_FIN,    /* Initiate graceful shutdown process. */	
 };