Fix ->d_lock locking order in unlazy_walk()

Make sure that child is still a child of parent before nested locking
of child->d_lock in unlazy_walk(); otherwise we are risking a violation
of locking order and deadlocks.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/namei.c

@@ -433,6 +433,8 @@ static int unlazy_walk(struct nameidata *nd, struct dentry *dentry)
 			goto err_parent;
 		BUG_ON(nd->inode != parent->d_inode);
 	} else {
+		if (dentry->d_parent != parent)
+			goto err_parent;
 		spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
 		if (!__d_rcu_to_refcount(dentry, nd->seq))
 			goto err_child;