d_ancestor
BUG_ON(d_ancestor(target, dentry));
p = d_ancestor(old_parent, target);
if (unlikely(d_ancestor(new, dentry))) {
subdir = d_ancestor(old_dentry, new_dentry);
subdir = d_ancestor(old_dentry, new_dentry);
extern struct dentry *d_ancestor(struct dentry *, struct dentry *);