vfs - fix dentry ref count in do_lookup()

There is a ref count problem in fs/namei.c:do_lookup().

When walking in ref-walk mode, if follow_managed() returns a fail we
need to drop dentry and possibly vfsmount.  Clean up properly,
as we do in the other caller of follow_managed().
Signed-off-by: Ian Kent <raven@themaw.net>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/namei.c

@@ -1272,8 +1272,10 @@ static int do_lookup(struct nameidata *nd, struct qstr *name,
 	path->mnt = mnt;
 	path->dentry = dentry;
 	err = follow_managed(path, nd->flags);
-	if (unlikely(err < 0))
+	if (unlikely(err < 0)) {
+		path_put_conditional(path, nd);
 		return err;
+	}
 	*inode = path->dentry->d_inode;
 	return 0;