cxgbe tom: Set the tid in the work requests to program page pods for iSCSI.

As a result, CPL_FW4_ACK now returns credits for these work requests.
To support this, page pod work requests are now constructed in special
mbufs similar to "raw" mbufs used for NIC TLS in plain TX queues.
These special mbufs are stored in the ulp_pduq and dispatched in order
with PDU work requests.

Sponsored by: Chelsio Communications
Discussed with: np
Differential Revision: https://reviews.freebsd.org/D29904