* Loss-making WeWork growing rapidly on demand for flexible leases. : SAN FRANCISCO/NEW YORK (Reuters) - Japan’s SoftBank Group Corp (9984.T) is in discussions to buy a majority stake in U.S. shared office space provider WeWork Cos, a source said, potentially doubling down on one of its biggest bets on a loss-making startup. A deal would signal a shift for SoftBank, which runs the world’s biggest private equity fund and has concentrated its ownership of late-stage startups in minority stakes, as it seeks attractive targets for its huge pool of capital.