blk-mq: protect debugfs_create_files() from failures

commit 36991ca68db9dd43bac7f3519f080ee3939263ef upstream.

If debugfs were to return a non-NULL error for a debugfs call, using
that pointer later in debugfs_create_files() would crash.

Fix that by properly checking the pointer before referencing it.
Reported-by: NMichal Hocko <mhocko@kernel.org>
Reported-and-tested-by: syzbot+b382ba6a802a3d242790@syzkaller.appspotmail.com
Reported-by: NTetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NJoseph Qi <joseph.qi@linux.alibaba.com>
Reviewed-by: NXiaoguang Wang <xiaoguang.wang@linux.alibaba.com>

block/blk-mq-debugfs.c

@@ -898,6 +898,9 @@ static const struct blk_mq_debugfs_attr blk_mq_debugfs_ctx_attrs[] = {
 static bool debugfs_create_files(struct dentry *parent, void *data,
 				 const struct blk_mq_debugfs_attr *attr)
 {
+	if (IS_ERR_OR_NULL(parent))
+		return false;
+
 	d_inode(parent)->i_private = data;
 
 	for (; attr->name; attr++) {