By Elizabeth Dilts Marshall WASHINGTON (Reuters) – U.S. banks are preparing for retaliatory cyber attacks after Western nations slapped a raft of stringent sanctions on Russia for invading Ukraine, cyber experts and executives said. Tensions between Russia and the West escalated on Saturday as the United States and its allies moved to block some Russian banks from the SWIFT international payment system and placed curbs on the Russian central bank’s international reserves. Western governments have warned for weeks that the tensions could spark massive cyber attacks from Russia or its supporters…