gfs2_convert: clear out old di_mode before setting it
    
    This patch clears the dinode 'di_mode' inode type field before
    setting it based on the GFS1 value. In rare circumstances
    (e.g. fsck.gfs2 ran on a GFS1 file system and created lost+found
    with both GFS1 and GFS2 data) di_mode might have an old value.
    In these cases, simply doing a logical 'or' with the correct
    value produces an invalid value. Zeroing it out beforehand ensures
    it is set correctly based on the GFS1 type.
    
    rhbz#887374