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>

net/sctp/protocol.c

@@ -1258,6 +1258,9 @@ SCTP_STATIC __init int sctp_init(void)
 	/* Set SCOPE policy to enabled */
 	sctp_scope_policy = SCTP_SCOPE_POLICY_ENABLE;
 
+	/* Set the default rwnd update threshold */
+	sctp_rwnd_upd_shift		= SCTP_DEFAULT_RWND_SHIFT;
+
 	sctp_sysctl_register();
 
 	INIT_LIST_HEAD(&sctp_address_families);