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

net/ipv6/netfilter/nf_nat_l3proto_ipv6.c

@@ -235,7 +235,7 @@ int nf_nat_icmpv6_reply_translation(struct sk_buff *skb,
 		inside->icmp6.icmp6_cksum =
 			csum_ipv6_magic(&ipv6h->saddr, &ipv6h->daddr,
 					skb->len - hdrlen, IPPROTO_ICMPV6,
-					csum_partial(&inside->icmp6,
+					skb_checksum(skb, hdrlen,
 						     skb->len - hdrlen, 0));
 	}