Zero checksum handling (#25)

Merge the compile warning fix  from the zero-csum branch:

* ISO C90 forbids mixed declarations and code [-Werror=declaration-after-statement]

Co-authored-by: Richard Patterson <richard@helix.net.nz>
Co-authored-by: Det <detobate@users.noreply.github.com>

nat46/modules/nat46-core.c

@@ -1576,12 +1576,13 @@ void nat46_ipv6_input(struct sk_buff *old_skb) {
         }
       case NEXTHDR_UDP: {
         struct udphdr *udp = add_offset(ip6h, v6packet_l3size);
+        u16 sum1, sum2;
         if ((udp->check == 0) && zero_csum_pass) {
           /* zero checksum and the config to pass it is set - do nothing with it */
           break;
         }
-        u16 sum1 = csum_ipv6_unmagic(nat46, &ip6h->saddr, &ip6h->daddr, l3_infrag_payload_len, NEXTHDR_UDP, udp->check);
-        u16 sum2 = csum_tcpudp_remagic(v4saddr, v4daddr, l3_infrag_payload_len, NEXTHDR_UDP, sum1);
+        sum1 = csum_ipv6_unmagic(nat46, &ip6h->saddr, &ip6h->daddr, l3_infrag_payload_len, NEXTHDR_UDP, udp->check);
+        sum2 = csum_tcpudp_remagic(v4saddr, v4daddr, l3_infrag_payload_len, NEXTHDR_UDP, sum1);
         udp->check = sum2;
         break;
         }