netfilter: Wrong icmp6 checksum for ICMPV6_TIME_EXCEED in reverse SNATv6 path

When recalculating the outer ICMPv6 checksum for a reverse path NATv6
such as ICMPV6_TIME_EXCEED nf_nat_icmpv6_reply_translation() was
accessing data beyond the headlen of the skb for non-linear skb.  This
resulted in incorrect ICMPv6 checksum as garbage data was used.

Patch replaces csum_partial() with skb_checksum() which supports
non-linear skbs similar to nf_nat_icmp_reply_translation() from ipv4.

Signed-off-by: Dave Johnson <dave-kernel@centerclick.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>