fsck.gfs2: don't remove buffers from the list when errors are found
    
    Before this patch, if an error was encountered while marking the
    data blocks, the blocks would be removed from the linked list.
    Now that we've got "undo" functions, we need to be able to undo
    the designations of those blocks, which means we need to keep those
    buffers on the linked list so they're found later. If we don't,
    the undo data block function won't process them, and therefore they'll
    be marked as "data" blocks in the bitmap, but no files will reference
    the blocks (because the error causes the inode to be deleted).
    With this patch, the metadata that points to the faulty data is kept
    on the linked list, and after the error is found, the undo function
    will therefore find it and mark its blocks as "free".
    
    rhbz#902920