block: check flags of claimed slave bdev to fix uaf for bd_holder_dir

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I60QE9


CVE: NA

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

As explained in 32c39e8a ("block: fix use after free for
bd_holder_dir"), we should make sure the "disk" is still live and
then grab a reference to 'bd_holder_dir'. However, the "disk"
should be "the claimed slave bdev" rather than "the holding disk".

Fixes: 32c39e8a ("block: fix use after free for bd_holder_dir")
Signed-off-by: Li Lingfeng <lilingfeng3@huawei.com>
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>