gfs2_convert: Fix rgrp conversion to allow re-converts
    
    Zero out gfs1 rgrp's rg_freemeta field after adding it
    to gfs2 rgrp's free blocks. This should only be done once.
    If convert fails or is interrupted and is restarted later
    on, adding a non-zero rg_freemeta to the free blocks again
    throws off the numbers and breaks allocation.