It's moments before that Goo Goo Dolls are set to perform on The Rosie O'Donnell Show, and lead singer and guitarist John Rzeznik has just thrown everyone out of his backstage dressing room. Behind the closed door he starts making low, gurgling sounds, followed by a sudden succession of screeching growls. Has the band's sudden success driven Rzeznik, well, goo-goo?Not exactly. Back at his hotel room after the show, Rzeznik (pronounced Rez-nik), 33, explains: "I have a cyst on my vocal cords from singing wrong. A vocal coach who works with everyone from Bjork to Pavorotti gave me a tape of exer