sctp: introduce round robin stream scheduler

This patch introduces RFC Draft ndata section 3.2 Priority Based
Scheduler (SCTP_SS_RR).

Works by maintaining a list of enqueued streams and tracking the last
one used to send data. When the datamsg is done, it switches to the next
stream.

See-also: https://tools.ietf.org/html/draft-ietf-tsvwg-sctp-ndata-13
Tested-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/sctp/stream_sched.c b/net/sctp/stream_sched.c
index 115ddb7..03513a9 100644
--- a/net/sctp/stream_sched.c
+++ b/net/sctp/stream_sched.c
@@ -122,10 +122,12 @@ static struct sctp_sched_ops sctp_sched_fcfs = {
 /* API to other parts of the stack */
 
 extern struct sctp_sched_ops sctp_sched_prio;
+extern struct sctp_sched_ops sctp_sched_rr;
 
 struct sctp_sched_ops *sctp_sched_ops[] = {
 	&sctp_sched_fcfs,
 	&sctp_sched_prio,
+	&sctp_sched_rr,
 };
 
 int sctp_sched_set_sched(struct sctp_association *asoc,