DATASN: We have to increment DATASN in each DATA-OUT PDU in a sequence.

For writes that span across multiple DATA-OUT segments we have to
increment the DATASN field for each segment.
We didnt use to, and most targets were perfectly happy, ignoring that the DATASN was constant 0 for every DATAOUT we sent.
LIO however does check this and did cause it to reject the libiscsi test tool.
(This is something we should write a iscsi test for  so we can flag all non-LIO targets as broken and needing to be fixed)

Signed-off-by: Ronnie Sahlberg <ronniesahlberg@gmail.com>

include/iscsi-private.h

@@ -182,6 +182,7 @@ struct iscsi_pdu {
 	uint32_t lun;
 	uint32_t itt;
 	uint32_t cmdsn;
+	uint32_t datasn;
 	enum iscsi_opcode response_opcode;
 
 	iscsi_command_cb callback;