Consider the two points \( P(\lambda, 2,-1) \) and \( Q(2,1,-1) \), where \( \lambda \) is a real number. Find all values of \( \lambda \) (if any) such that the angle between \( \overrightarrow{Q P} \) and the vector \( \mathbf{u}=\langle 0,1,1\rangle \) is \( \frac{\pi}{3} \).