ATHENS (Reuters) - Greek banks are expected to have covered a shortfall in initial bids by the time Athens closes an extended offer on Tuesday to buy back bonds as part of an effort to cut debt and secure aid for its flagging economy. Athens gave bondholders until 7 a.m. EDT on Tuesday to tender Greek debt after the amount offered by Friday's initial deadline fell short of the targeted 30 billion euros. The cut-price buyback's success is crucial to putting Greece's debt back on a sustainable footing, which in turn allows lenders to disburse aid to Athens. ...