we use the flag ISCSI_PDU_NO_CALLBACK and pdu->callback simultaneously, but
check only for one of them in various places. So drop ISCSI_PDU_NO_CALLBACK
and check for pdu->callback != NULL instead.
All PDUs that carried this flag have pdu->callback set to NULL.
Signed-off-by: Peter Lieven <pl@kamp.de>
cde2043891