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

hulk inclusion
category: bugfix
bugzilla: 186386, https://gitee.com/openeuler/kernel/issues/I520OX


CVE: NA
backport: openEuler-22.03-LTS

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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")
Signed-off-by: Zhang Wensheng <zhangwensheng5@huawei.com>
Reviewed-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>