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/Makefile b/net/sctp/Makefile
index 647c9cf..bf90c53 100644
--- a/net/sctp/Makefile
+++ b/net/sctp/Makefile
@@ -12,7 +12,8 @@
 	  inqueue.o outqueue.o ulpqueue.o \
 	  tsnmap.o bind_addr.o socket.o primitive.o \
 	  output.o input.o debug.o stream.o auth.o \
-	  offload.o stream_sched.o stream_sched_prio.o
+	  offload.o stream_sched.o stream_sched_prio.o \
+	  stream_sched_rr.o
 
 sctp_probe-y := probe.o