nbd: fix max value for 'first_minor'

stable inclusion
from stable-v5.10.170
commit fd8107206a672f5a529a819e8930bf81fb54ae0f
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I7V9QX

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fd8107206a672f5a529a819e8930bf81fb54ae0f



----------------------------------------------------

commit e4c4871a upstream.

commit b1a81163 ("block: nbd: add sanity check for first_minor")
checks that 'first_minor' should not be greater than 0xff, which is
wrong. Whitout the commit, the details that when user pass 0x100000,
it ends up create sysfs dir "/sys/block/43:0" are as follows:

nbd_dev_add
 disk->first_minor = index << part_shift
  -> default part_shift is 5, first_minor is 0x2000000
  device_add_disk
   ddev->devt = MKDEV(disk->major, disk->first_minor)
    -> (0x2b << 20) | (0x2000000) = 0x2b00000
   device_add
    device_create_sys_dev_entry
	 format_dev_t
	  sprintf(buffer, "%u:%u", MAJOR(dev), MINOR(dev));
	   -> got 43:0
	  sysfs_create_link -> /sys/block/43:0

By the way, with the wrong fix, when part_shift is the default value,
only 8 ndb devices can be created since 8 << 5 is greater than 0xff.

Since the max bits for 'first_minor' should be the same as what
MKDEV() does, which is 20. Change the upper bound of 'first_minor'
from 0xff to 0xfffff.

Fixes: b1a81163 ("block: nbd: add sanity check for first_minor")
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Link: https://lore.kernel.org/r/20211102015237.2309763-2-yebin10@huawei.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Wen Yang <wenyang.linux@foxmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: zhaoxiaoqiang11 <zhaoxiaoqiang11@jd.com>