Fix gfs2_fsck segfault
    
    bz 496330
    
    The gfs2_fsck tool was segfaulting if gfs2 had any blocks
    assigned to two different purposes.  There were two problems.
    First, struct blocks was not big enough to handle the data
    that pass1b needed for duplicate processing.  Second,
    function pass1b() was improperly referencing the duplicate
    blocks list.