sctp: Update SWS avaoidance receiver side algorithm

We currently send window update SACKs every time we free up 1 PMTU
worth of data.  That a lot more SACKs then necessary.  Instead, we'll
now send back the actuall window every time we send a sack, and do
window-update SACKs when a fraction of the receive buffer has been
opened.  The fraction is controlled with a sysctl.

Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>
diff --git a/net/sctp/sm_sideeffect.c b/net/sctp/sm_sideeffect.c
index eda4fe7..8ae6709 100644
--- a/net/sctp/sm_sideeffect.c
+++ b/net/sctp/sm_sideeffect.c
@@ -217,8 +217,7 @@
 		sctp_add_cmd_sf(commands, SCTP_CMD_TIMER_RESTART,
 				SCTP_TO(SCTP_EVENT_TIMEOUT_SACK));
 	} else {
-		if (asoc->a_rwnd > asoc->rwnd)
-			asoc->a_rwnd = asoc->rwnd;
+		asoc->a_rwnd = asoc->rwnd;
 		sack = sctp_make_sack(asoc);
 		if (!sack)
 			goto nomem;