hopefully get the subreq task_fini dance right now

Ref: eb805e8e

we cannot signal task_fini after posting subreq.done, because then we
race the assertion that all tasks are done when we are done delivering.

But we also cannot do it the other way around because then the
assertion that subreqs are done when all tasks are finished, does not
hold.

So the right option should be to do both under the tree lock.

src/vdp_pesi.c

@@ -217,8 +217,8 @@ vped_task(struct worker *wrk, void *priv)
 	case T_SUBREQ:
 		assert (node->type == T_SUBREQ);
 		assert(pesi == req->transport_priv);
-		task_fini(pesi_tree, pesi);
 		Lck_Lock(&pesi_tree->tree->tree_lock);
+		task_fini(pesi_tree, pesi);
 		node->subreq.done = 1;
 		AZ(pthread_cond_signal(&node->subreq.cond));
 		Lck_Unlock(&pesi_tree->tree->tree_lock);