When “The Karate Kid” came out in the summer of 1984, the little movie about rival martial arts students, competing against big-budget blockbusters, became one of the highest-grossing films of the year. Ralph Macchio and his co-star William Zabka have since transferred their young rivalry into adulthood, by playing the grown-up Daniel LaRusso and Johnny Lawrence in the series “Cobra Kai,” now in its sixth and final season on Netflix. For Macchio it’s a return to a character that has both enriched his life and challenged it. Correspondent Lee Cowan reports.