block: restore a specific error code in bdev_del_partition

mdadm relies on the fact that deleting an invalid partition returns
-ENXIO or -ENOTTY to detect if a block device is a partition or a
whole device.

Fixes: 08fc1ab6 ("block: fix locking in bdev_del_partition")
Reported-by: kernel test robot <rong.a.chen@intel.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>