““A return to classical bank policy would deem loans fraudulent and annul debts when creditors do not lend with any reasonable calculation of how the debt can be paid in the normal course of economic life. Loans made without such a calculation should be considered predatory. The natural check on such behavior is to permit mortgage debtors to walk away from their homes, free of the debts attached to them, letting title revert to the banks that over-lent.””