gfs: improve gfs_fsck rindex repair code
    
    bz 442271 - GFS: gfs_fsck bugs found in rindex repair code
    
    This patch makes improvements and fixes some bugs in gfs_fsck's
    rindex repair code.  Basically, if RGs are damaged, especially
    in the third section (i.e. RGs added by gfs_grow) it did not
    properly locate the RG boundaries.  Also, if the rindex was
    completely zeroed out, it did not recover it in cases where
    the file system had been extended.