vxlan: Fixed checksum caclculation offset

VXLAN uses csum_offload for IPv6 packets.

But without gso node we have csum calculated only for inner
packet.
This patch adds support for outer header csum calculation.
Checksum for inner packet should be calculated before
interface-output node (for example in vxlan node).

Type: fix

Signed-off-by: Mohsin Kazmi <sykazmi@cisco.com>
Signed-off-by: Vladimir Isaev <visaev@netgate.com>
Change-Id: Ica68429ede4426293769207cd83c791ebe72fe56

test/test_vxlan_gpe.py

@@ -80,6 +80,8 @@ class TestVxlanGpe(BridgeDomain, VppTestCase):
         # Verify UDP destination port is VXLAN-GPE 4790, source UDP port
         #  could be arbitrary.
         self.assertEqual(pkt[UDP].dport, type(self).dport)
+        # Verify UDP checksum
+        self.assert_udp_checksum_valid(pkt)
         # Verify VNI
         self.assertEqual(pkt[VXLAN].vni, vni)