bdi: get device name under rcu protect

hulk inclusion
category: bugfix
bugzilla: 30109
CVE: NA
---------------------------

bdi->dev may be set as "NULL" or freed by bdi_unregister().
To avoid causing "NULL" pointer reference or use-after-free
in user, we add a common function bdi_get_dev_name(), in which
dev is protected by RCU lock. Then, the caller can get device
name safely.

Fixes: 5ca4579ae59b ("bdi: fix use-after-free for the bdi device")
Signed-off-by: Yufen Yu <yuyufen@huawei.com>
Reviewed-by: Hou Tao <houao1@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>