nbd: Add the maximum limit of allocated index in nbd_dev_add

If the index allocated by idr_alloc greater than MINORMASK >> part_shift,
the device number will overflow, resulting in failure to create a block
device.

Fix it by imiting the size of the max allocation.

Signed-off-by: Zhong Jinghua <zhongjinghua@huawei.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20230605122159.2134384-1-zhongjinghua@huaweicloud.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>