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: 668c9be ("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>

include/net/sctp/sm.h

@@ -347,7 +347,7 @@ static inline __u16 sctp_data_size(struct sctp_chunk *chunk)
 	__u16 size;
 
 	size = ntohs(chunk->chunk_hdr->length);
-	size -= sctp_datahdr_len(&chunk->asoc->stream);
+	size -= sctp_datachk_len(&chunk->asoc->stream);
 
 	return size;
 }