target: move ref_cmd from the generic se_tmr_req into iscsi code

Also remove the unused ref_task_lun field in struct se_tmr_req.

(nab: Add missing TASK_REASSIGN ref_lun vs. ref_cmd orig_fe_lun checks
      in iscsit_tmr_task_reassign)

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>