fsck.gfs2: Fix declaration mismatch for invalidate_*
    
    Move the invalidate_fxns struct definition down so that the forward
    declarations for those functions can be removed to avoid mismatches.
    
    Fixes sparse error:
    
      pass1.c:1146:12: error: symbol 'invalidate_data' redeclared with
      different type (incompatible argument 6 (different base types))
    
    Signed-off-by: Andrew Price <anprice@redhat.com>