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>