sctp: adjust ssthresh when transport is idle

sctp: adjust ssthresh when transport is idle

RFC 4960 Errata 3.27 identifies that ssthresh should be adjusted to cwnd
because otherwise it could cause the transport to lock into congestion
avoidance phase specially if ssthresh was previously reduced by some
packet drop, leading to poor performance.

The Errata says to adjust ssthresh to cwnd only once, though the same
goal is achieved by updating it every time we update cwnd too. The
caveat is that we could take longer to get back up to speed but that
should be compensated by the fact that we don't adjust on RTO basis (as
RFC says) but based on Heartbeats, which are usually way longer.

See-also: https://tools.ietf.org/html/draft-ietf-tsvwg-rfc4960-errata-01#section-3.27
Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>