Use the right lock for signalling subreq.done

This looks like a trivial oversight in 3370a4c4,
which was meant to fix #10: We need to use the corresponding lock to signal
when a subreq node is done.

Hopefully fixes #13

src/vdp_pesi.c

@@ -232,11 +232,11 @@ vped_task(struct worker *wrk, void *priv)
 		break;
 	case T_SUBREQ:
 		assert(pesi == req->transport_priv);
-		Lck_Lock(&pesi_tasks->bytes_tree->tree_lock);
+		Lck_Lock(node->subreq.shared_lock);
 		task_fini(pesi_tasks, pesi);
 		node->subreq.done = 1;
 		AZ(pthread_cond_signal(&node->subreq.cond));
-		Lck_Unlock(&pesi_tasks->bytes_tree->tree_lock);
+		Lck_Unlock(node->subreq.shared_lock);
 		break;
 	case T_NEXUS:
 		assert(node->nexus.req == req);