fsck.gfs2: Verify rgrps free space against bitmap
    
    This patch makes fsck.gfs2 check the actual bits in the rgrp
    bitmaps to see how many blocks are free.  It verifies that
    value against the rg_free value in the rgrp.  It needs to
    ensure that it starts with an accurate value for rg_free because
    an inaccurate value can get us into trouble and cause fsck.gfs2
    to abort abnormally.  This can happen in cases where it tries to
    allocate blocks (for example, for lost+found metadata) when the
    bitmaps say there is free space when there really isn't.
    
    rhbz#455300