Lawmakers on Capitol Hill have said they're going to take a look at the fees Fannie Mae and Freddie Mac charge lenders for guaranteeing mortgages. And judging from a recent discussion of the subject by industry experts, this could mean more trouble ahead for the GSEs. Debate over the so-called G-fee has simmered for years, as lenders big and small have quietly groused about a fee structure that they claim is out of line with…