This patch against recent mainline bk removes the bogus WARN_ON for
single_lun devices, and a meaningless comment.

We clear the starget_sdev_user, and immediately blk_run_queue for the LUN
that just issued IO. Another LUN could race in scsi_request_fn, but it is
most likely that the last user will get there first, and reset
starget_sdev_user. If it does not, it will have to wait for the other LUN
to finish all of its IO.