libgfs2: Move bitmap buffers inside struct gfs2_bitmap
    
    Keeping an array of buffers alongside the array of bitmaps meant some
    extra management and bookkeeping for arrays of buffer pointers. Move the
    buffer pointers into the bitmap structures.
    
    The only downside to this is that reading resource groups gets a little
    more complicated and an intermediate array of buffer pointers is created
    in gfs2_rgrp_read to make sure we can still use vector i/o. Fortunately
    this doesn't cause any noticeable slowdown.
    
    Signed-off-by: Andrew Price <anprice@redhat.com>