sanlock: don't clear the shared flag on disk
    
    When a shared lease is acquired, the shared flag is
    written to the mode_block (following the dblock structure),
    and then the leader record is released.
    
    In commit e40e1f6e22f9b10f08d53fc7da94f5158d9e4ae8,
    releasing the leader record also began to clear the
    dblock structure.  Clearing the dblock also wrongly
    cleared the mode_block, causing the shared flag to
    be lost.  This means that sanlock believes a shared
    lease is held, when in fact on disk it is not held,
    and can be acquired exclusively by another host.