ub: use __blk_end_request_all()

ub_end_rq() always tries to complete full request.  The @cmd_len
parameter was there because rq->data_len used to be overwritten with
residue count.  Drop @cmd_len and use __blk_end_request_all().

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Pete Zaitcev <zaitcev@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>