U.S. stocks traded about 2 percent lower on Friday, extending a recent rout, as concerns about slowing global growth continued to pressure investor sentiment. ( Tweet This ) The Dow Jones industrial average briefly fell into correction territory, losing more than 470 points as all blue chips declined. In the last five years, the index has only had four instances with closing losses of more than 400 points. "It's an expiration day and it looks like they're to have for sale on the close maybe as much as a billion dollars," said Art Cashin, director of floor trading for UBS....