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

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>