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/test_mtu.py

@@ -101,7 +101,6 @@ class TestMTU(VppTestCase):
                           dst=self.pg0.remote_ip4,
                           ttl=254, len=576, id=0) /
                        p_icmp4 / p_ip4 / p_payload)
-        icmp4_reply[1].ttl -= 1
         n = icmp4_reply.__class__(icmp4_reply)
         s = bytes(icmp4_reply)
         icmp4_reply = s[0:576]
@@ -118,7 +117,7 @@ class TestMTU(VppTestCase):
         p4 = p_ether / p_ip4 / p_payload
         p4.flags = 0
         p4_reply = p_ip4 / p_payload
-        p4_reply.ttl = 62  # check this
+        p4_reply.ttl = p_ip4.ttl - 1
         p4_reply.flags = 0
         p4_reply.id = 256
         self.pg_enable_capture()