Uber’s early backers are on a collision course with their own greed. Japan’s SoftBank and allied investors want to buy roughly one-sixth of the bumptious ride-hailing firm. Existing owners can sell at a $48 billion valuation, or about 30 percent below the last one. Even with a markdown, they could make many times their money. The promise of more, however risky, will cloud the thinking. Consider just one of the many rounds of funds raised under now-deposed co-founder Travis Kalanick. A so-called Series B investment in 2011 judged Uber to be worth $300 million before an injection of $37 million of