UPSTREAM: fs: erofs: add sanity check for kobject in erofs_unregister_sysfs

[ Upstream commit a942da24 ]

Syzkaller hit 'WARNING: kobject bug in erofs_unregister_sysfs'. This bug
is triggered by injecting fault in kobject_init_and_add of
erofs_unregister_sysfs.

Fix this by adding sanity check for kobject in erofs_unregister_sysfs

Note that I've tested the patch and the crash does not occur any more.

Link: https://lore.kernel.org/r/20220315132814.12332-1-dzm91@hust.edu.cn


Change-Id: I8d26b9f82758d8c0bbb2832150de2f289e17fe2b
Signed-off-by: Dongliang Mu <mudongliangabcd@gmail.com>
Fixes: 168e9a76 ("erofs: add sysfs interface")
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Reported-by:  <syzbot+4d15e77deaec58116d46@syzkaller.appspotmail.com>
Fixes: 3450010a ("UPSTREAM: erofs: add sysfs interface")
Signed-off-by: Jun Nie <jun.nie@linaro.org>