target/iscsi: Misc cleanups from Agrover (round 2)

This patch includes the handful of squashed patches for target/iscsi from
Andy's original series into lio-core/master code:

*) Make iscsit_add_reject static
*) Remove unused data_offset_end from iscsi_datain_req
*) Remove "#if 0" stubs
*) Rename iscsi_datain_req to cmd_datain_node
*) Cleanups for built_r2ts_for_cmd()
*) Cleanups for Cleanup build_sendtargets_response()

Signed-off-by: Andy Grover <agrover@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>