fsck.gfs2: standardize check_metatree return codes
    
    This patch aims to not change functionality at all. What it does is
    adds a standard set of three return codes with the following meanings:
    
    meta_is_good - all is well, keep processing metadata normally
    meta_skip_further - an non-fatal error occurred, so further metadata
                        processing for this inode should be skipped.
    meta_error - a fatal error occurred in this metadata, so we need to
                 abort processing.
    
    rhbz#902920