Nick Barnett, the Green Bay Packers' leading tackler in three of his four seasons with the club, on Tuesday agreed to a six-year contract extension, a deal that will keep him off the free agent market next spring. Financial details of the extension were not yet available, but it is likely the guarantees in the new deal total at least $10 million. The extension culminates months of discussions between the two sides, with the negotiations having been ratcheted up in recent days.