sctp: Try not to change a_rwnd when faking a SACK from SHUTDOWN.

We currently set a_rwnd to 0 when faking a SACK from SHUTDOWN.
This results in an hung association if the remote only uses
SHUTDOWNs (which it's allowed to do) to acknowlege DATA when
closing.  The reason for that is that we simply honor the a_rwnd
from the sack, but since we faked it to be 0, we enter 0-window
probing.  The fix is to use the peers old rwnd and add our flight
size to it.
Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>

net/sctp/sm_sideeffect.c

@@ -1533,7 +1533,8 @@ static int sctp_cmd_interpreter(sctp_event_t event_type,
 		case SCTP_CMD_PROCESS_CTSN:
 			/* Dummy up a SACK for processing. */
 			sackh.cum_tsn_ack = cmd->obj.be32;
-			sackh.a_rwnd = 0;
+			sackh.a_rwnd = asoc->peer.rwnd +
+					asoc->outqueue.outstanding_bytes;
 			sackh.num_gap_ack_blocks = 0;
 			sackh.num_dup_tsns = 0;
 			sctp_add_cmd_sf(commands, SCTP_CMD_PROCESS_SACK,
@@ -1632,9 +1633,9 @@ static int sctp_cmd_interpreter(sctp_event_t event_type,
 	 */
 	if (asoc && SCTP_EVENT_T_CHUNK == event_type && chunk) {
 		if (chunk->end_of_packet || chunk->singleton)
-			sctp_outq_uncork(&asoc->outqueue);
+			error = sctp_outq_uncork(&asoc->outqueue);
 	} else if (local_cork)
-			sctp_outq_uncork(&asoc->outqueue);
+		error = sctp_outq_uncork(&asoc->outqueue);
 	return error;
 nomem:
 	error = -ENOMEM;