Exclude reserved iSCSI Initiator Task Tag.
RFC 7143 (11.2.1.8):
An ITT value of 0xffffffff is reserved and MUST NOT be assigned for a task by the initiator. The only instance in which it may be seen on the wire is in a target-initiated NOP-In PDU (Section 11.19) and in the initiator response to that PDU, if necessary.
MFC after: 1 month