This function can allocate a new pdu using a specific itt value
and specific flags instead of using the defaults of, next iff, no flags set.
This can be used when we need to allocate additional PDUs in a chain,
for commands that span across multiple PDUs and where all need to keep
the same itt value.
For example
->WRITE10 cdb
->DATAOUT
-<RESPONSE
Here the DATAOUT PDU belongs to the same task that was started by write10
so it need to use the same itt value.
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