ip: Fragmentation fixes

Type: fix

if the packet is about to be fragmented, then don't call any of the
actions that expect the rewrite to have been written.
1) don't double count packets thru the adjacency (original & fragments)
2) don't double decrement the TTL for fragments
3) return to ip4-midchain post ip-frag if that's where we started.
4) only run midchain/mcast fixups if not fragmenting (if no errors)

Change-Id: Ib2866787a42713ee5871b87b597d8f74b901044b
Signed-off-by: Neale Ranns <nranns@cisco.com>

test/vpp_neighbor.py

@@ -58,6 +58,7 @@ class VppNeighbor(VppObject):
             flags=self.flags)
         self.stats_index = r.stats_index
         self._test.registry.register(self, self._test.logger)
+        return self
 
     def remove_vpp_config(self):
         self._test.vapi.ip_neighbor_add_del(