BOSTON — Celtics coach Brad Stevens has said what has carried his team through the NBA postseason is its ability to make “winning plays” with the game on the line. And more times than not, it’s been Marcus Smart making them. Since returning from thumb surgery late in the first round, Smart has been unflappable and Boston’s unquestioned deacon of toughness. From sacrificing his body diving for loose balls or keeping plays alive with a rebound , to causing deflections on defense, to popping up out of the crowd for a timely rebound or tip-in, Smart is the proud everyman for a once underdog Celtics team that has outworked the Cavaliers through the first two games of the Eastern Conference finals. “That’s me.