nbd: fix assignment error for first_minor in nbd_dev_add

hulk inclusion
category: bugfix
bugzilla: 188413, https://gitee.com/openeuler/kernel/issues/I6GWYG


CVE: NA

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A panic error is like below:

nbd_genl_connect
 nbd_dev_add
   first_minor = index << part_shift; // index =-1
   ...
   __device_add_disk
     blk_alloc_devt
       *devt = MKDEV(disk->major, disk->first_minor + part->partno);
       // part->partno = 0， first_minor = 11...110000 major is covered

There, index < 0 will reassign an index, but here disk->first_minor is
assigned -1 << part_shift.

This causes to the creation of the device with the same major and minor
device numbers each time the incoming index<0, and this will lead to
creation of kobject failed:
Warning: kobject_add_internal failed for 4095:1048544 with -EEXIST, don't
try to register things with the same name in the same directory.

Fix it by moving the first_minor assignment down to after getting the new
index.

Fixes: 01f7594e ("nbd: Fix use-after-free in blk_mq_free_rqs")
Signed-off-by: Zhong Jinghua <zhongjinghua@huawei.com>
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>