A heroic ROTC student fatally stabbed the crazed gunman who opened fire inside an Old Dominion University classroom Thursday, preventing further carnage, law enforcement sources said. Shooter Mohamed Bailor Jalloh, 36, gunned down an instructor before the unidentified cadet jumped into action during the frantic scene on the Virginia college campus, according to sources. Jalloh, who was previously convicted of trying to support ISIS, walked into the classroom and asked if it was an ROTC class, sources told The Post. When someone told him it was, he began his trail of bloodshed, which the FBI is investigating as an act...