net/packet/checksum: fix v6 NAT

We were copying 12 out of the 16 bytes which meant that
the 1:1 NAT required would only work if the last 4 bytes
happened to match between the new and old address, something
that our tests accidentally had. Fix it by copying the full
16 bytes and make the tests also verify the addr and use rand
addresses.

Updates #9511

Signed-off-by: Maisem Ali <maisem@tailscale.com>

net/packet/checksum/checksum.go

@@ -61,7 +61,7 @@ func UpdateDstAddr(q *packet.Parsed, dst netip.Addr) {
 	b := q.Buffer()
 	if dst.Is6() {
 		v6 := dst.As16()
-		copy(b[24:36], v6[:])
+		copy(b[24:40], v6[:])
 		updateV6PacketChecksums(q, old, dst)
 	} else {
 		v4 := dst.As4()