fsck.gfs2: Use O_DIRECT
    
    This required changes in libgfs2 and fsck.gfs2 to make sure buffers are
    appropriately aligned. posix_fadvise()-based readahead is also removed.
    
    This adds a significant overhead where buffered i/o was previously
    beneficial. This should be accounted for in future improvements.
    
    Signed-off-by: Andrew Price <anprice@redhat.com>