sctp: Add support to per-association statistics via a new SCTP_GET_ASSOC_STATS call

The current SCTP stack is lacking a mechanism to have per association
statistics. This is an implementation modeled after OpenSolaris'
SCTP_GET_ASSOC_STATS.

Userspace part will follow on lksctp if/when there is a general ACK on
this.
V4:
- Move ipackets++ before q->immediate.func() for consistency reasons
- Move sctp_max_rto() at the end of sctp_transport_update_rto() to avoid
  returning bogus RTO values
- return asoc->rto_min when max_obs_rto value has not changed

V3:
- Increase ictrlchunks in sctp_assoc_bh_rcv() as well
- Move ipackets++ to sctp_inq_push()
- return 0 when no rto updates took place since the last call

V2:
- Implement partial retrieval of stat struct to cope for future expansion
- Kill the rtxpackets counter as it cannot be precise anyway
- Rename outseqtsns to outofseqtsns to make it clearer that these are out
  of sequence unexpected TSNs
- Move asoc->ipackets++ under a lock to avoid potential miscounts
- Fold asoc->opackets++ into the already existing asoc check
- Kill unneeded (q->asoc) test when increasing rtxchunks
- Do not count octrlchunks if sending failed (SCTP_XMIT_OK != 0)
- Don't count SHUTDOWNs as SACKs
- Move SCTP_GET_ASSOC_STATS to the private space API
- Adjust the len check in sctp_getsockopt_assoc_stats() to allow for
  future struct growth
- Move association statistics in their own struct
- Update idupchunks when we send a SACK with dup TSNs
- return min_rto in max_rto when RTO has not changed. Also return the
  transport when max_rto last changed.

Signed-off: Michele Baldessari <michele@acksyn.org>
Acked-by: Vlad Yasevich <vyasevich@gmail.com>

Signed-off-by: David S. Miller <davem@davemloft.net>

net/sctp/transport.c

@@ -363,6 +363,7 @@ void sctp_transport_update_rto(struct sctp_transport *tp, __u32 rtt)
 	if (tp->rto > tp->asoc->rto_max)
 		tp->rto = tp->asoc->rto_max;
 
+	sctp_max_rto(tp->asoc, tp);
 	tp->rtt = rtt;
 
 	/* Reset rto_pending so that a new RTT measurement is started when a
@@ -620,6 +621,7 @@ void sctp_transport_reset(struct sctp_transport *t)
 	t->burst_limited = 0;
 	t->ssthresh = asoc->peer.i.a_rwnd;
 	t->rto = asoc->rto_initial;
+	sctp_max_rto(asoc, t);
 	t->rtt = 0;
 	t->srtt = 0;
 	t->rttvar = 0;