BRUSSELS — The European Commission is warning national governments not to ban the export of fuel across internal EU borders, with the Iran war threatening to choke oil supply to European refineries. The closure of the Strait of Hormuz, a vital waterway between Iran and Oman, has affected 20 percent of global oil flows, threatening shortfalls in some European refineries. That has prompted some countries to consider emergency export restrictions, with Hungary and Slovakia moving last month to ban