cxgbei: Support iSCSI offload on T6.

T6 makes several changes relative to T5 for receive of iSCSI PDUs.

First, earlier adapters issue either 2 or 3 messages to the host for
each PDU received: CPL_ISCSI_HDR contains the BHS of the PDU,
CPL_ISCSI_DATA (when DDP is not used for zero-copy receive) contains
the PDU data as buffers on the freelist, and CPL_RX_ISCSI_DDP with
status of the PDU such as result of CRC checks. In T6, a new
CPL_RX_ISCSI_CMP combines CPL_ISCSI_HDR and CPL_RX_ISCSI_DDP. Data
PDUs which are directly placed via DDP only report a single
CPL_RX_ISCSI_CMP message. Data PDUs received on the free lists are
reported as CPL_ISCSI_DATA followed by CPL_RX_ISCSI_CMP. Control PDUs
such as R2T are still reported via CPL_ISCSI_HDR and CPL_RX_ISCSI_DDP.

Supporting this requires changing the CPL_ISCSI_DATA handler to
allocate a PDU structure if it is not preceded by a CPL_ISCSI_HDR as
well as support for the new CPL_RX_ISCSI_CMP.

Second, when using DDP for zero-copy receive, T6 will only issue a
CPL_RX_ISCSI_CMP after a burst of PDUs have been received (indicated
by the F flag in the BHS). In this case, the CPL_RX_ISCSI_CMP can
reflect the completion of multiple PDUs and the BHS and TCP sequence
number included in the message are from the last PDU received in the
burst. Notably, the message does not include any information about
earlier PDUs received as part of the burst. Instead, the driver must
track the amount of data already received for a given transfer and use
this to compute the amount of data received in a burst. In addition,
the iSCSI layer currently has no way to permit receiving a logical PDU
which spans multiple PDUs. Instead, the driver presents each burst as
a single, "large" PDU to the iSCSI target and initiators. This is
done by rewriting the buffer offset and data length fields in the BHS
of the final PDU as well as rewriting the DataSN so that the received
PDUs appear to be in order.

To track all this, cxgbei maintains a hash table of 'cxgbei_cmp'
structures indexed by transfer tags for each offloaded iSCSI
connection. When a SCSI_DATA_IN message is received, the ITT from the
received BHS is used to find the necessary state in the hash table,
whereas SCSI_DATA_OUT replies use the TTT as the key. The structure
tracks the expected starting offset and DataSN of the next burst as
well as the rewritten DataSN value used for the previously received
PDU.

Discussed with: np
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D30458