sctp: add basic structures and make chunk function for idata

sctp_idatahdr and sctp_idata_chunk are used to define and parse
I-DATA chunk format, and sctp_make_idata is a function to build
the chunk.

The I-DATA Chunk Format is defined in section 2.1 of RFC8260.
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/sctp.h

@@ -243,6 +243,23 @@ struct sctp_data_chunk {
 	struct sctp_datahdr data_hdr;
 };
 
+struct sctp_idatahdr {
+	__be32 tsn;
+	__be16 stream;
+	__be16 reserved;
+	__be32 mid;
+	union {
+		__u32 ppid;
+		__be32 fsn;
+	};
+	__u8 payload[0];
+};
+
+struct sctp_idata_chunk {
+	struct sctp_chunkhdr chunk_hdr;
+	struct sctp_idatahdr data_hdr;
+};
+
 /* DATA Chuck Specific Flags */
 enum {
 	SCTP_DATA_MIDDLE_FRAG	= 0x00,

include/net/sctp/sm.h

@@ -197,6 +197,8 @@ struct sctp_chunk *sctp_make_cookie_ack(const struct sctp_association *asoc,
 struct sctp_chunk *sctp_make_cwr(const struct sctp_association *asoc,
 				 const __u32 lowest_tsn,
 				 const struct sctp_chunk *chunk);
+struct sctp_chunk *sctp_make_idata(const struct sctp_association *asoc,
+				   __u8 flags, int paylen, gfp_t gfp);
 struct sctp_chunk *sctp_make_datafrag_empty(struct sctp_association *asoc,
 					    const struct sctp_sndrcvinfo *sinfo,
 					    int len, const __u8 flags,

include/net/sctp/structs.h

@@ -575,6 +575,7 @@ struct sctp_chunk {
 		struct sctp_addiphdr *addip_hdr;
 		struct sctp_fwdtsn_hdr *fwdtsn_hdr;
 		struct sctp_authhdr *auth_hdr;
+		struct sctp_idatahdr *idata_hdr;
 	} subh;
 
 	__u8 *chunk_end;

net/sctp/sm_make_chunk.c

@@ -1425,6 +1425,12 @@ static struct sctp_chunk *sctp_make_data(const struct sctp_association *asoc,
 	return _sctp_make_chunk(asoc, SCTP_CID_DATA, flags, paylen, gfp);
 }
 
+struct sctp_chunk *sctp_make_idata(const struct sctp_association *asoc,
+				   __u8 flags, int paylen, gfp_t gfp)
+{
+	return _sctp_make_chunk(asoc, SCTP_CID_I_DATA, flags, paylen, gfp);
+}
+
 static struct sctp_chunk *sctp_make_control(const struct sctp_association *asoc,
 					    __u8 type, __u8 flags, int paylen,
 					    gfp_t gfp)