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: NXin Long <lucien.xin@gmail.com>
Acked-by: NMarcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Acked-by: NNeil Horman <nhorman@tuxdriver.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>