An after-party for Will Smith’s critically maligned “After Earth” turned into a DJ Jazzy Jeff & the Fresh Prince reunion when Smith’s former musical collaborator spun early ’90s rap, and his co-star from the sitcom “The Fresh Prince of Bel-Air” Alfonso Ribeiro jumped onstage and started “going nuts,” spies said. “Alfonso was getting down to Michael Jackson. He was doing the robot and the ‘Carlton Banks’ dance [from ‘Fresh Prince’],” said a witness. While Ribeiro “took his jacket off mid-performance,” Smith and wife Jada watched from afar, but their son Jaden was at the side of the stage and looked like he wanted to get in on the act. But “Alfonso was just going for it,” said a source. Smith, Jazzy, Jaden and Ribeiro recently danced on Graham Norton’s UK show. But despite the nostalgia trip, Ribeiro said of a reunion of his ’90s NBC show, “I think people should let that show live in the past . . . we’ll have these fun little reunions every time we get together.” Smith’s daughter Willow was spotted at the Mercedes-Benz-sponsored party, exclaiming, “This is so dope!”