Mary Carillo wasn’t supposed to be in the booth for the Milan-Cortina Opening Ceremonies Friday. She was supposed to be prepping some other Olympic segments, or reminiscing about her best Real Sports investigations, or scouting a mixed-doubles tennis tournament in Timbuktu, or whatever it is Mary Carillo does when she’s not in front of the camera. Instead, she’ll be in the booth with Shaun White and Terry Gannon, watching the Parade of Nations and commenting on all those Mariah Carey melismas, gamely stepping in while original host Savannah Guthrie stays in the US. to attend to the tragic disappearance of her mom. Who is Mary Carillo? If you’re a sports fan, you already know. But if you’re one of the millions who’s tuning in Friday just for the pageantry and don’t know your curling from your Kazakh mogul specialists, here’s the skinny on the Brooklyn-born, Florida-residing American treasure. She’s mad sardonic. Tennis fans know Carillo for her wry comments, often delivered with a world-class deadpan, leaving only alert viewers to pick up the joke. Once when the Spanish …