fsck.gfs2: Add find_remove_dup, free_block_if_notdup
    
    This patch adds a couple new functions to util.c: find_remove_dup
    and free_block_if_notdup.  This is a centralized function that
    is used to remove a duplicate reference.  Each reference needs to be
    removed to determine when all duplicate references have been resolved.
    The object is to get down to one reference, so we can set the proper
    type based on the remaining reference.  However, if the very last
    reference is deleted as well, the block may be freed.
    These functions present one consistent way to do all this rather than
    before when the it wasn't very consistent and various bitmap
    inaccuracies were sometimes left in the file system.
    
    rhbz#675723