netfilter: ipv6: fix use-after-free Write in nf_nat_ipv6_manip_pkt

l4proto->manip_pkt() can cause reallocation of skb head so pointer
to the ipv6 header must be reloaded.

Reported-and-tested-by:  <syzbot+10005f4292fc9cc89de7@syzkaller.appspotmail.com>
Fixes: 58a317f1 ("netfilter: ipv6: add IPv6 NAT support")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>