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

Since nf_flow_snat_port and nf_flow_snat_ip{v6} call pskb_may_pull()
which 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>