block: Fix an error handling in add_partition

Once we have called device_initialize(), we should use put_device() to
give up the reference on error, just like what we have done on failure
of device_add().

Signed-off-by: Dinghao Liu <dinghao.liu@zju.edu.cn>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>