netfilter: use nf_ip6_check_hbh_len in nf_ct_skb_network_trim

For IPv6 Jumbo packets, the ipv6_hdr(skb)->payload_len is always 0,
and its real payload_len ( > 65535) is saved in hbh exthdr. With 0
length for the jumbo packets, all data and exthdr will be trimmed
in nf_ct_skb_network_trim().

This patch is to call nf_ip6_check_hbh_len() to get real pkt_len
of the IPv6 packet, similar to br_validate_ipv6().

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Reviewed-by: Aaron Conole <aconole@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>