ovl: don't fail with disconnected lower NFS

 * Check if this layer root is a descendant of:

 * - another layer of this overlayfs instance

 * - upper/work dir of any overlayfs instance

 * - a disconnected dentry (detached root)

 */

static int ovl_check_layer(struct super_block *sb, struct dentry *dentry,

			   const char *name)

{

	struct dentry *next, *parent;

	bool is_root = false;

	struct dentry *next = dentry, *parent;

	int err = 0;

	if (!dentry || dentry == dentry->d_sb->s_root)

	if (!dentry)

		return 0;

	next = dget(dentry);

	/* Walk back ancestors to fs root (inclusive) looking for traps */

	do {

	parent = dget_parent(next);

		is_root = (parent == next);

	/* Walk back ancestors to root (inclusive) looking for traps */

	while (!err && parent != next) {

		if (ovl_is_inuse(parent)) {

			err = -EBUSY;

			pr_err("overlayfs: %s path overlapping in-use upperdir/workdir\n",

			err = -ELOOP;

			pr_err("overlayfs: overlapping %s path\n", name);

		}

		dput(next);

		next = parent;

	} while (!err && !is_root);

	/* Did we really walk to fs root or found a detached root? */

	if (!err && next != dentry->d_sb->s_root) {

		err = -ESTALE;

		pr_err("overlayfs: disconnected %s path\n", name);

		parent = dget_parent(next);

		dput(next);

	}

	dput(next);

	dput(parent);

	return err;

}