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>

cxgbit: add files for cxgbit.ko

cxgbit.h - This file contains data structure
definitions for cxgbit.ko.

cxgbit_lro.h - This file contains data structure
definitions for LRO support.

cxgbit_main.c - This file contains code for
registering with iscsi target transport and
cxgb4 driver.

cxgbit_cm.c - This file contains code for
connection management.

cxgbit_target.c - This file contains code
for processing iSCSI PDU.

cxgbit_ddp.c - This file contains code for
Direct Data Placement.

(added check for NULL sg in cxgbit_set_one_ppod)
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>

(add Kconfig and Makefile v2: added dependency on INET)
Reported-by: Arnd Bergmann <arnd@arndb.de>

Signed-off-by: Varun Prakash <varun@chelsio.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>