Btrfs-progs: don't output baffling message when checking a fresh fs
    
    As we know,  a new fs doesn't have space cache, so we set the cache generation
    of the super block to be -1ULL, it is not equal to the fs generation. But the
    check program didn't consider this case, and output the following message
    
      cache and super generation don't match, space cache will be invalidated
    
    directly, it would be baffling the users. So we should avoid outputing such
    message. This patch fixes this problem.
    
    Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
    Signed-off-by: David Sterba <dsterba@suse.cz>