[SCSI] libfc, tcm_fc: add ddp_targ() to libfc function template to supprot FCoE DDP in target mode

The fcoe driver can implement ddp_targ() similarly to ddp_setup() when fcoe
stack works with existing target frame, e.g., tcm, where the ddp_targ() would
eventually point to the underlying hardware driver's implementation of
ndo_fcoe_ddp_targ() through net_device_ops. This new API sets up DDP context
for target appropriately by setting required bits for DDP context.

Signed-off-by: Yi Zou <yi.zou@intel.com>
Signed-off-by: Kiran Patil <kiran.patil@intel.com>
Signed-off-by: Robert Love <robert.w.love@intel.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>