Spurred on by a desire to invest a growing amount of U.S. dollars in fixed-income investments, foreign buying of mortgage backed securities is apparently growing at a very brisk pace in 2005. According to new estimates compiled by Credit Suisse First Boston, overseas purchases of U.S. MBS soared to over $80 billion during the first quarter of this year. That represented the highest level of buying seen since at least 2000 and possibly the highest…