sctp: fix the data size calculation in sctp_data_size

sctp data size should be calculated by subtracting data chunk header's
length from chunk_hdr->length, not just data header.

Fixes: 668c9beb

 ("sctp: implement assign_number for sctp_stream_interleave")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>