nbd: add sanity check for first_minor

hulk inclusion
category: bugfix
bugzilla: 182920, https://gitee.com/openeuler/kernel/issues/I4GLNX


CVE: NA
---------------------------

When user pass 0x100000 as index, nbd will end up create sysfs dir
"/sys/block/43:0":

nbd_dev_add
 disk->first_minor = index << part_shift
 -> default part_shift is 5, 0x100000 << 5 = 0x2000000
  device_add_disk
   blk_alloc_devt
    MKDEV(disk->major, disk->first_minor + part->partno)
    -> (0x2b << 20) | (0x2000000) = 0x2b00000
   register_disk
    device_add
     device_create_sys_dev_entry
      format_dev_t
       MAJOR(devt) -> 0x2b00000 >> 20 = 0x2b
       MINOR(devt) -> 0x2b00000 & 0xfffff = 0
      sysfs_create_link -> /sys/block/43:0

If nbd created device with index 0 aready, then sysfs will compalin
about dumplicated creation.

On the other hand, the similar dumplicated creation will happen if
"index << part_shift" over flow to a value that is less than MINORMASK.

Thus fix the problem by adding sanity check for first_minor.

Fixes: b0d9111a ("nbd: use an idr to keep track of nbd devices")
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>