The average long-term U.S. mortgage rate came off its lowest level in three and a half years this week, as bond yields marched higher following a spike in oil prices due to the war with Iran . The benchmark 30-year fixed rate mortgage rate ticked up to 6% from 5.98% last week, mortgage buyer Freddie Mac said Thursday. One year ago, the rate averaged 6.63%. The modest increase ends a three-week slide in the average rate, which has been hovering around 6% this year. Last week’s average rate