nbd: Fix incorrect error handle when first_minor is illegal in nbd_dev_add

If first_minor is illegal will goto out_free_idr label, this will miss
cleanup disk.

Fixes: b1a81163 ("block: nbd: add sanity check for first_minor")
Signed-off-by: Ye Bin <yebin10@huawei.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Link: https://lore.kernel.org/r/20211102015237.2309763-4-yebin10@huawei.com


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