sctp: introduce sctp_chunk_stream_no

Add a helper to fetch the stream number from a given chunk.
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>

include/net/sctp/structs.h

@@ -642,6 +642,11 @@ void sctp_init_addrs(struct sctp_chunk *, union sctp_addr *,
 		     union sctp_addr *);
 const union sctp_addr *sctp_source(const struct sctp_chunk *chunk);
 
+static inline __u16 sctp_chunk_stream_no(struct sctp_chunk *ch)
+{
+	return ntohs(ch->subh.data_hdr->stream);
+}
+
 enum {
 	SCTP_ADDR_NEW,		/* new address added to assoc/ep */
 	SCTP_ADDR_SRC,		/* address can be used as source */