iscsi-target: add void (*iscsit_get_r2t_ttt)()

Add void (*iscsit_get_r2t_ttt)() to
struct iscsit_transport, iscsi-target
uses this callback to get
r2t->targ_xfer_tag.

cxgbit.ko needs this callback
for Direct Data Placement of Data Out
pdus, adapter uses ttt in Data Out pdus
for placing data directly in to the host
buffers.

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