Bayern Munich coach Julian Nagelsmann has vowed changes after the freshly crowned Bundesliga champions delivered a listless performance in a 3-1 defeat at Mainz. And Hertha Berlin coach Felix Magath urged an instant turnaround and more professionalism in Munich next weekend against VfB Stuttgart, who are fighting relegation along with his team. Bayern’s meltdown in Mainz was their fifth league defeat in a season which also includes a shocking 5-0 German Cup loss at Borussia Mönchengladbach and a surprise quarter-final exit in the Champions League against Villarreal. “There have been too many d…