Student-loan borrowers may be racking up billions of dollars in extra debt needlessly. That’s one conclusion from a report on consumer credit released Monday by personal-finance site NerdWallet. Roughly 2.6 million borrowers with an average balance of $43,538 were in forbearance on their federal student loans last quarter — a status that allows borrowers to pause payments, but where interest still builds, according to NerdWallet’s analysis of government data.