ublk_drv: fix ublk device leak in case that add_disk fails

->free_disk is only called after disk is added successfully, so
drop ublk device reference in case of add_disk() failure.

Fixes: 6d9e6dfd ("ublk: defer disk allocation")
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20220730092750.1118167-3-ming.lei@redhat.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/ublk_drv.c

@@ -1190,6 +1190,11 @@ static int ublk_ctrl_start_dev(struct io_uring_cmd *cmd)
 	get_device(&ub->cdev_dev);
 	ret = add_disk(disk);
 	if (ret) {
+		/*
+		 * Has to drop the reference since ->free_disk won't be
+		 * called in case of add_disk failure.
+		 */
+		ublk_put_device(ub);
 		put_disk(disk);
 		goto out_unlock;
 	}