IPIP and IPv6 fragmentation

- Error where ICMPv6 error code doesn't reset VLIB_TX = -1
  Leading to crash for ICMP generated on tunnelled packets
- Missed setting VNET_BUFFER_F_LOCALLY_ORIGINATED, so
  IP in IPv6 packets never got fragmented.
- Add support for fragmentation of buffer chains.
- Remove support for inner fragmentation in frag code itself.

Change-Id: If9a97301b7e35ca97ffa5c0fada2b9e7e7dbfb27
Signed-off-by: Ole Troan <ot@cisco.com>

test/test_mpls.py

@@ -360,7 +360,7 @@ class TestMPLS(VppTestCase):
                 # ICMP sourced from the interface's address
                 self.assertEqual(rx_ip.src, src_if.local_ip6)
                 # hop-limit reset to 255 for IMCP packet
-                self.assertEqual(rx_ip.hlim, 254)
+                self.assertEqual(rx_ip.hlim, 255)
 
                 icmp = rx[ICMPv6TimeExceeded]