sctp: implement prsctp RTX policy

prsctp RTX policy is a policy to abandon chunks when they are
retransmitted beyond the max count.

This patch uses sent_count to count how many times one chunk has
been sent, and prsctp_param is the max rtx count, which is from
sinfo->sinfo_timetolive in sctp_set_prsctp_policy(). So similar
to TTL policy, if RTX policy is enabled, msg->expire_at won't
work.

Then in sctp_chunk_abandoned, this patch checks if chunk->sent_count
is bigger than chunk->prsctp_param to abandon this chunk.
Signed-off-by: NXin Long <lucien.xin@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/sctp/chunk.c

@@ -355,6 +355,10 @@ int sctp_chunk_abandoned(struct sctp_chunk *chunk)
 		else
 			chunk->asoc->abandoned_unsent[SCTP_PR_INDEX(TTL)]++;
 		return 1;
+	} else if (SCTP_PR_RTX_ENABLED(chunk->sinfo.sinfo_flags) &&
+		   chunk->sent_count > chunk->prsctp_param) {
+		chunk->asoc->abandoned_sent[SCTP_PR_INDEX(RTX)]++;
+		return 1;
 	}
 
 	return 0;

net/sctp/sm_make_chunk.c

@@ -720,6 +720,8 @@ static void sctp_set_prsctp_policy(struct sctp_chunk *chunk,
 	if (SCTP_PR_TTL_ENABLED(sinfo->sinfo_flags))
 		chunk->prsctp_param =
 			jiffies + msecs_to_jiffies(sinfo->sinfo_timetolive);
+	else if (SCTP_PR_RTX_ENABLED(sinfo->sinfo_flags))
+		chunk->prsctp_param = sinfo->sinfo_timetolive;
 }
 
 /* Make a DATA chunk for the given association from the provided