Commercial banks are at it again, adding billions of mortgage securities to their investment portfolios and defying predictions that the industry would lose interest in the MBS market. According to data analyzed by Inside MBS & ABS, commercial banks held a record $910.69 billion of residential mortgage securities on their books at the end of the first quarter. That was up 3.9 percent in just three months, or a projected 15.6 percent annual rate, and…