By Tanner Garrity The very condition that prompted Will Smith to whack Chris Rock across the face a couple months ago — alopecia areata — may soon be a thing of the past. Massachusetts-based lab Concert Pharmaceuticals reported this week that its drug CTP-543 can restore a head of hair in up to 42% of people with the extreme hair loss condition. The results arrived after an extensive study that included 700 volunteers — all of whom had lost at least half of their hair, while some came in completely bald. The subjects were instructed to take the pill twice a day, and after six months, an encour…