ublk: fail to start device if queue setup is interrupted

In ublk_ctrl_start_dev(), if wait_for_completion_interruptible() is
interrupted by signal, queues aren't setup successfully yet, so we
have to fail UBLK_CMD_START_DEV, otherwise kernel oops can be triggered.

Reported by German when working on qemu-storage-deamon which requires
single thread ublk daemon.

Fixes: 71f28f31 ("ublk_drv: add io_uring based userspace block driver")
Reported-by: German Maglione <gmaglione@redhat.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20230726144502.566785-2-ming.lei@redhat.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/ublk_drv.c

@@ -1847,7 +1847,8 @@ static int ublk_ctrl_start_dev(struct ublk_device *ub, struct io_uring_cmd *cmd)
 	if (ublksrv_pid <= 0)
 		return -EINVAL;
 
-	wait_for_completion_interruptible(&ub->completion);
+	if (wait_for_completion_interruptible(&ub->completion) != 0)
+		return -EINTR;
 
 	schedule_delayed_work(&ub->monitor_work, UBLK_DAEMON_MONITOR_PERIOD);