selftests/ublk: fix error handling for starting device

selftests/ublk: fix error handling for starting device

Fix error handling in ublk_start_daemon() when start_dev fails:

1. Call ublk_ctrl_stop_dev() to cancel inflight uring_cmd before
   cleanup. Without this, the device deletion may hang waiting for
   I/O completion that will never happen.

2. Add fail_start label so that pthread_join() is called on the
   error path. This ensures proper thread cleanup when startup fails.

Fixes: 6aecda00b7d1 ("selftests: ublk: add kernel selftests for ublk")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Caleb Sander Mateos <csander@purestorage.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>