zebra: Only notify dplane work pthread when needed

The fpm_nl_process function was getting the count
of the total number of ctx's processed.  This leads
to after having processed 1 context to always signal
the dataplane that there is work to do.  Change the
code to only notify the dplane worker when a context
was actually added to the outgoing context queue.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>

zebra/dplane_fpm_nl.c

@@ -1525,7 +1525,7 @@ static void fpm_process_queue(struct event *t)
 	 * until the dataplane thread gets scheduled for new,
 	 * unrelated work.
 	 */
-	if (dplane_provider_out_ctx_queue_len(fnc->prov) > 0)
+	if (processed_contexts)
 		dplane_provider_work_ready();
 }