From celebration to concern. Former Knick J.R. Smith got knocked to the ground during the postgame celebration outside of Madison Square Garden on Monday night in a hectic scene, with the folk hero sternly telling those around him, “Relax, man, relax,” after the Knicks’ 137-98 win over the 76ers in Game 1. Smith, who played for the team from 2011-2015, often finds his way back to The Garden to watch big Knicks games, and he took to the streets after the blowout win to start this second-round series with a smile while smoking something rolled up in a white casing. The scenes outside Knicks playoffs games are always hectic , especially after wins but even after losses, and fans quickly mobbed Smith while he ventured into the mass of humanity. The situation quickly changed, though, with Smith — and others — being knocked from his feet and having to be helped back to an upright position several moments later. Smith smiling while smoking as he enters the mosh pit.…