ublk: return -EINTR if breaking from waiting for existed users in DEL_DEV

If user interrupts wait_event_interruptible() in ublk_ctrl_del_dev(),
return -EINTR and let user know what happens.

Fixes: 0abe39d ("block: ublk: improve handling device deletion")
Reported-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20230726144502.566785-4-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Ming Lei

drivers/block/ublk_drv.c

@@ -2126,8 +2126,8 @@ static int ublk_ctrl_del_dev(struct ublk_device **p_ub)
 	 * - the device number is freed already, we will not find this
 	 *   device via ublk_get_device_from_id()
 	 */
-	wait_event_interruptible(ublk_idr_wq, ublk_idr_freed(idx));
-
+	if (wait_event_interruptible(ublk_idr_wq, ublk_idr_freed(idx)))
+		return -EINTR;
 	return 0;
 }