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>

include/net/sctp/sctp.h

@@ -272,6 +272,18 @@ struct sctp_mib {
         unsigned long   mibs[SCTP_MIB_MAX];
 };
 
+/* helper function to track stats about max rto and related transport */
+static inline void sctp_max_rto(struct sctp_association *asoc,
+				struct sctp_transport *trans)
+{
+	if (asoc->stats.max_obs_rto < (__u64)trans->rto) {
+		asoc->stats.max_obs_rto = trans->rto;
+		memset(&asoc->stats.obs_rto_ipaddr, 0,
+			sizeof(struct sockaddr_storage));
+		memcpy(&asoc->stats.obs_rto_ipaddr, &trans->ipaddr,
+			trans->af_specific->sockaddr_len);
+	}
+}
 
 /* Print debugging messages.  */
 #if SCTP_DEBUG