【OLK-5.10】block: fix use after free for bd_holder_dir

Currently, the caller of bd_link_disk_holer() get 'bdev' by
blkdev_get_by_dev(), which will look up 'bdev' by inode number 'dev'.
Howerver, it's possible that del_gendisk() can be called currently, and
'bd_holder_dir' can be freed before bd_link_disk_holer() access it, thus
use after free is triggered.

t1: t2:
bdev = blkdev_get_by_dev
del_gendisk
┊kobject_put(bd_holder_dir)
┊ kobject_free()
bd_link_disk_holder

Fix the problem by checking disk is still live and grabbing a reference
to 'bd_holder_dir' first in bd_link_disk_holder().

Link: https://lore.kernel.org/all/20221103025541.1875809-3-yukuai1@huaweicloud.com/

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