zebra: add structure to hold per-prefix state in RIB

Add the rib_dest_t structure to hold per-prefix state in the routing
information base. This gives us an appropriate place to maintain the
queueing state of a route_node. Queuing state was previously being
stored on the first rib in the list of ribs hanging off the
route_node.

  * zebra/rib.h

    - Add new structure rib_dest_t.

    - Remove the rn_status field from 'struct rib', it is no longer
      required.

    - Add macros (RNODE_FOREACH_RIB, RNODE_FOREACH_RIB_SAFE) for
      walking all 'struct ribs' corresponding to a route_node. These
      hide the fact that there is an intermediate rib_dest_t
      structure.

    - Add a few utility inlines to go between a rib_dest_t and
      associated structures.

  * zebra/zebra_rib.c

    - rib_link()/rib_unlink()

      Tweak for new behavior, where the 'info' pointer of a route_node
      points to a rib_dest_t. The list of ribs for a prefix now hangs
      off of the dest.

      Change the way we ref count route_nodes. We now hold a single
      ref count on a route_node if there is a corresponding
      rib_dest_t.

    - Maintain the queuing state of a route_node on the flags field of
      the rib_dest_t.

    - Add the rib_gc_dest() function, which deletes a rib_dest_t if it
      is no longer required. A rib_dest_t can be deleted iff there are
      no struct ribs hanging off of it.

    - Call rib_gc_dest() any time we unlink a rib from the
      rib_dest_t. Currently we only need to call it once, just before
      we return from rib_process().

  * zebra/{redistribute,zebra_rib,zebra_snmp,zebra_vty}.c

    Use new macros to walk over route_node ribs.

  * lib/memtypes.c

    Add memory type for rib_dest_t.

Signed-off-by: Avneesh Sachdev <avneesh@opensourcerouting.org>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/zebra/redistribute.c b/zebra/redistribute.c
index 4276f1d..4765824 100644
--- a/zebra/redistribute.c
+++ b/zebra/redistribute.c
@@ -107,7 +107,7 @@
       rn = route_node_lookup (table, (struct prefix *)&p);
       if (rn)
 	{
-	  for (newrib = rn->info; newrib; newrib = newrib->next)
+	  RNODE_FOREACH_RIB (rn, newrib)
 	    if (CHECK_FLAG (newrib->flags, ZEBRA_FLAG_SELECTED)
 		&& newrib->distance != DISTANCE_INFINITY)
 	      zsend_route_multipath (ZEBRA_IPV4_ROUTE_ADD, client, &rn->p, newrib);
@@ -127,7 +127,7 @@
       rn = route_node_lookup (table, (struct prefix *)&p6);
       if (rn)
 	{
-	  for (newrib = rn->info; newrib; newrib = newrib->next)
+	  RNODE_FOREACH_RIB (rn, newrib)
 	    if (CHECK_FLAG (newrib->flags, ZEBRA_FLAG_SELECTED)
 		&& newrib->distance != DISTANCE_INFINITY)
 	      zsend_route_multipath (ZEBRA_IPV6_ROUTE_ADD, client, &rn->p, newrib);
@@ -148,7 +148,7 @@
   table = vrf_table (AFI_IP, SAFI_UNICAST, 0);
   if (table)
     for (rn = route_top (table); rn; rn = route_next (rn))
-      for (newrib = rn->info; newrib; newrib = newrib->next)
+      RNODE_FOREACH_RIB (rn, newrib)
 	if (CHECK_FLAG (newrib->flags, ZEBRA_FLAG_SELECTED) 
 	    && newrib->type == type 
 	    && newrib->distance != DISTANCE_INFINITY
@@ -159,7 +159,7 @@
   table = vrf_table (AFI_IP6, SAFI_UNICAST, 0);
   if (table)
     for (rn = route_top (table); rn; rn = route_next (rn))
-      for (newrib = rn->info; newrib; newrib = newrib->next)
+      RNODE_FOREACH_RIB (rn, newrib)
 	if (CHECK_FLAG (newrib->flags, ZEBRA_FLAG_SELECTED)
 	    && newrib->type == type 
 	    && newrib->distance != DISTANCE_INFINITY