After more than a decade of singing about his divorce and how much the music industry sucks, Tom Petty returns to his roots on Mojo, his first album with the Heartbreakers in eight years. He and the group build most of the 15 songs around the bar-band blues they honed during their formative days. For fans of Southern Accents throwaway “Spike,” this might be good news, but for everyone else, Mojo is just as dull as The Last DJ and Echo. It’s Petty’s least forced album in years (most of it was recorded live and without overdubs), and it sounds blithely tossed-off. But that’s a problem. Many songs come off like concert set-fillers made by a band that doesn’t have enough material to fill an entire night. Even the album’s best tracks — “Jefferson Jericho Blues,” “I Should Have Known It,” and “Good Enough” — sound like they were written so Petty and the Heartbreakers have something new to jam to onstage. And anyone who’s sat through a Phish record knows how tedious that can be.