Sources say Tyrann Mathieu publicly urged Lane Kiffin to bring Bryce Underwood back, calling on LSU to re-sign the QB who had committed to joining the team before leaving for a huge sum from the sale of the player named after him. But behind the scenes, that request reportedly angered Kiffin. According to those close to the matter, Kiffin sent Mathieu a short, cold 15-word message…