selftests: ublk: make ublk_stop_io_daemon() more reliable

selftests: ublk: make ublk_stop_io_daemon() more reliable

Improve ublk_stop_io_daemon() in the following ways:

- don't wait if ->ublksrv_pid becomes -1, which means that the disk
has been stopped

- don't wait if ublk char device doesn't exist any more, so we can
avoid to rely on inoitfy for wait until the char device is closed

And this way may reduce time of delete command a lot.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Link: https://lore.kernel.org/r/20250303124324.3563605-2-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>