fsck.gfs2: Don't check fs formats we don't recognise
    
    Currently fsck.gfs2 will ignore sb_fs_format but in order to support
    future formats we need to make sure it doesn't try to check filesystems
    with formats we don't recognise yet. Better late than never.
    
    Tests included.
    
    rhbz#1616389
    rhbz#1622050
    
    Signed-off-by: Andrew Price <anprice@redhat.com>