set the cmdsn in the pdu struct so we can compare with
maxcmdsn when sending to socket even if data-out pdus
do not carry a cmdsn on the wire.
if we would not set pdu->cmdsn comparsion with
iscsi->maxcmdsn would cause a deadlock after
maxcmdsn increases to 2^31.
Signed-off-by: Peter Lieven <pl@kamp.de>