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>