o  rte can now contain a pointer to both cached and uncached rta. Protocols
     which don't need their own attribute caching can now just fill-in a rta,
     link it to rte without any calls to attribute cache and call rte_update()
     which will replace rte->attrs by a cached copy.

  o  In order to support this, one of previously pad bytes in struct rta
     now holds new attribute flags (RTAF_CACHED). If you call rte_update()
     with uncached rta, you _must_ clear these flags. In other cases rta_lookup()
     sets it appropriately.

  o  Added rte_free() which is useful when you construct a rte and then the
     circumstances change and you decide not to use it for an update. (Needed
     for temporary rte's in kernel syncer...)