TOKYO -- Japanese stocks have been wildly gyrating due to uncertainty over China's economy and as hedge funds have engaged in short-term trading. While the Bank of Japan is expected to buttress sagging stock prices by upping its purchases of exchange-traded funds, it may be running out of money for this purpose.      As part of its quantitative and qualitative easing policy, the BOJ each year purchases 3 trillion yen ($24.6 billion) worth of ETFs linked to Japanese stocks. With less than four months left this year, the bank could be expected to spend the rest of 2015 on...