tayga: fix broken ICMP checksum on big-endian machines

This patches fixes wrong ICMP checksum of translated packets on
big-endian machines #16715
The patch is authored by upstream author
Nathan Lutchansky <lutchann@litech.org>
Source of the patch: http://forum.mikrotik.com/viewtopic.php?f=15&t=82329

Signed-off-by: Ondrej Caletka <ondrej@caletka.cz>

ipv6/tayga/patches/002-bigendian_wrong_checksum.patch

@@ -0,0 +1,53 @@
+--- a/nat64.c
++++ b/nat64.c
+@@ -19,6 +19,11 @@
+ 
+ extern struct config *gcfg;
+ 
++static uint16_t checksum_extend_byte(uint8_t b)
++{
++	return htons(b << 8);
++}
++
+ static uint16_t ip_checksum(void *d, int c)
+ {
+ 	uint32_t sum = 0xffff;
+@@ -30,7 +35,7 @@ static uint16_t ip_checksum(void *d, int
+ 	}
+ 
+ 	if (c)
+-		sum += htons(*((uint8_t *)p) << 8);
++		sum += checksum_extend_byte(*((uint8_t *)p));
+ 
+ 	while (sum > 0xffff)
+ 		sum = (sum & 0xffff) + (sum >> 16);
+@@ -180,10 +185,12 @@ static int xlate_payload_4to6(struct pkt
+ 		cksum = ones_add(p->icmp->cksum, cksum);
+ 		if (p->icmp->type == 8) {
+ 			p->icmp->type = 128;
+-			p->icmp->cksum = ones_add(cksum, ~(128 - 8));
++			p->icmp->cksum = ones_add(cksum,
++						~checksum_extend_byte(128 - 8));
+ 		} else {
+ 			p->icmp->type = 129;
+-			p->icmp->cksum = ones_add(cksum, ~(129 - 0));
++			p->icmp->cksum = ones_add(cksum,
++						~checksum_extend_byte(129 - 0));
+ 		}
+ 		return 0;
+ 	case 17:
+@@ -668,10 +675,12 @@ static int xlate_payload_6to4(struct pkt
+ 		cksum = ones_add(p->icmp->cksum, cksum);
+ 		if (p->icmp->type == 128) {
+ 			p->icmp->type = 8;
+-			p->icmp->cksum = ones_add(cksum, 128 - 8);
++			p->icmp->cksum = ones_add(cksum,
++						checksum_extend_byte(128 - 8));
+ 		} else {
+ 			p->icmp->type = 0;
+-			p->icmp->cksum = ones_add(cksum, 129 - 0);
++			p->icmp->cksum = ones_add(cksum,
++						checksum_extend_byte(129 - 0));
+ 		}
+ 		return 0;
+ 	case 17: