Each ublk queue is started before adding disk, we have to cancel queues in
ublk_stop_dev() so that ubq daemon can be exited, otherwise DEL_DEV command
may hang forever.

Also avoid to cancel queues two times by checking if queue is ready,
otherwise use-after-free on io_uring may be triggered because ublk_stop_dev
is called by ublk_remove() too.

Fixes: 71f28f31 ("ublk_drv: add io_uring based userspace block driver")
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20220730092750.1118167-2-ming.lei@redhat.comSigned-off-by: Jens Axboe <axboe@kernel.dk>