nbd: fix possible overflow on 'first_minor' in nbd_dev_add()

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2083917



commit 858f1bf6
Author: Zhang Wensheng <zhangwensheng5@huawei.com>
Date:   Sat May 21 15:37:48 2022 +0800

    nbd: fix possible overflow on 'first_minor' in nbd_dev_add()

    When 'index' is a big numbers, it may become negative which forced
    to 'int'. then 'index << part_shift' might overflow to a positive
    value that is not greater than '0xfffff', then sysfs might complains
    about duplicate creation. Because of this, move the 'index' judgment
    to the front will fix it and be better.

    Fixes: b0d9111a ("nbd: use an idr to keep track of nbd devices")
    Fixes: 940c2649 ("nbd: fix possible overflow for 'first_minor' in nbd_dev_add()")
Signed-off-by: Zhang Wensheng <zhangwensheng5@huawei.com>
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
    Link: https://lore.kernel.org/r/20220521073749.3146892-6-yukuai3@huawei.com


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

Signed-off-by: Ming Lei <ming.lei@redhat.com>