PPPoE usses a midchain adjacency stack on an interface-tx DPO

1) introduce an interface-tx DPO. This is a simple wrapper around a sw_if_index. enhance DPO stacking functions to allow per-instance next-nodes and hence allow children to stack onto the interface per-instance tx node and not on 'interface-output'.
2) update PPPoE code to use ta midchain stack on a interface-tx DPO of the encap-interface. This remove the need for pppoe_encap node (which is replaced by the adj-midchain-tx) and interface-output node is no longer used (see above). Since PPPoE encap node is no longer needed, the PPPoE seesion does not need to be retrieved in the data-path, hence the cahce misses are removed.

Change-Id: Id8b40f53daa14889a9c51d802e14fed7fba4399a
Signed-off-by: Neale Ranns <nranns@cisco.com>
diff --git a/src/vnet/fib/fib_path.c b/src/vnet/fib/fib_path.c
index 58050cc..cf5a463 100644
--- a/src/vnet/fib/fib_path.c
+++ b/src/vnet/fib/fib_path.c
@@ -21,7 +21,7 @@
 #include <vnet/dpo/receive_dpo.h>
 #include <vnet/dpo/load_balance_map.h>
 #include <vnet/dpo/lookup_dpo.h>
-#include <vnet/dpo/interface_dpo.h>
+#include <vnet/dpo/interface_rx_dpo.h>
 #include <vnet/dpo/mpls_disposition.h>
 
 #include <vnet/adj/adj.h>
@@ -1707,9 +1707,9 @@
 	/*
 	 * Resolve via a receive DPO.
 	 */
-	interface_dpo_add_or_lock(path->fp_nh_proto,
-                                  path->intf_rx.fp_interface,
-                                  &path->fp_dpo);
+	interface_rx_dpo_add_or_lock(path->fp_nh_proto,
+                                     path->intf_rx.fp_interface,
+                                     &path->fp_dpo);
 	break;
     }
     case FIB_PATH_TYPE_EXCLUSIVE:
@@ -2041,9 +2041,9 @@
             /*
              * Create the adj needed for sending IP multicast traffic
              */
-            interface_dpo_add_or_lock(fib_forw_chain_type_to_dpo_proto(fct),
-                                      path->attached.fp_interface,
-                                      dpo);
+            interface_rx_dpo_add_or_lock(fib_forw_chain_type_to_dpo_proto(fct),
+                                         path->attached.fp_interface,
+                                         dpo);
             break;
         case FIB_PATH_TYPE_RECEIVE:
         case FIB_PATH_TYPE_SPECIAL: