ofproto-dpif: Handle learn action flow mods asynchronously.

Once we have multiple threads running, having each execute flow mods
created by the learn action won't be tenable.  It essentially will
require us to make the core ofproto module thread safe, which is not
the direction we want to go.  This patch punts on the problem by
handing flow mods to ofproto-dpif to handle later.

Signed-off-by: Ethan Jackson <ethan@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>

lib/ofp-util.h

@@ -212,6 +212,8 @@ struct ofpbuf *ofputil_make_flow_mod_table_id(bool flow_mod_table_id);
  * The handling of cookies across multiple versions of OpenFlow is a bit
  * confusing.  See DESIGN for the details. */
 struct ofputil_flow_mod {
+    struct list list_node;    /* For queuing flow_mods. */
+
     struct match match;
     unsigned int priority;