loop: return proper error from loop_queue_rq()

commit b4a567e8 upstream.

->queue_rq() should return one of the BLK_MQ_RQ_QUEUE_* constants, not
an errno.

Fixes: f4aa4c7b ("block: loop: convert to per-device workqueue")
Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>