netfilter: flowtable: reload ip{v6}h in nf_flow_tuple_ip{v6}

Since pskb_may_pull may change skb->data, so we need to reload ip{v6}h at
the right place.

Fixes: a908fdec ("netfilter: nf_flow_table: move ipv6 offload hook code to nf_flow_table")
Fixes: 7d208687

 ("netfilter: nf_flow_table: move ipv4 offload hook code to nf_flow_table")
Signed-off-by: Haishuang Yan <yanhaishuang@cmss.chinamobile.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>