target/cxgbit: add T6 iSCSI DDP completion feature

Chelsio T6 adapters reduce number of completion
to host by generating single completion for all
directly placed(DDP) iSCSI pdus in a sequence,
completion contains iSCSI hdr of the last pdu
in a sequence.

On receiving DDP completion cxgbit driver finds
iSCSI cmd using iscsit_find_cmd_from_itt_or_dump(),
then updates cmd->write_data_done, cmd->next_burst_len,
cmd->data_sn and calls __iscsit_check_dataout_hdr()
to validate iSCSI hdr.

(Update __iscsit_check_dataout_hdr parameter usage - nab)

Signed-off-by: Varun Prakash <varun@chelsio.com>
Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>