csum_replace
csum_replace(&skb->csum, (__force __wsum)(old_ipv6_tclass << 12),
csum_replace(&skb->csum, (__force __wsum)htonl(ofl), (__force __wsum)htonl(fl));
csum_replace(&skb->csum, (__force __wsum)(nh->hop_limit << 8),