bgpd: "Intern" communities in route maps

* bgp_community.[ch]: (community_lookup) New helper function to look
  up a community list in the hash table.

* bgp_routemap.c: A new community structure was being allocated for
  every BGP update which matched a route map which set a community.
  This behavior led to rapid growth in the memory consumed by bgpd.
  Adding the communities to the hash table addresses the memory
  growth, but may introduce a problem in modifying or deleting the
  'set community' statement in the route map.
diff --git a/bgpd/bgp_community.c b/bgpd/bgp_community.c
index ae1d7a1..68383ad 100644
--- a/bgpd/bgp_community.c
+++ b/bgpd/bgp_community.c
@@ -292,6 +292,13 @@
   return str;
 }
 
+/* Find an 'intern'ed community structure */
+struct community *
+community_lookup (struct community *com)
+{
+   return (struct community *) hash_lookup (comhash, com);
+}
+
 /* Intern communities attribute.  */
 struct community *
 community_intern (struct community *com)