register the worker which is unpending

because the push phase of unpending happens outside the tree lock, we
need to ensure only one thread is ever entering unpending

this is in preparation for future changes, right now, unpending is only
ever called from the topreq thread

src/vdp_pesi.c

@@ -246,6 +246,7 @@ struct bytes_tree {
 	pthread_cond_t		cond;
 	struct node		*root;
 	struct node		*front;
+	struct worker		*unpend_owner;
 	int			npending;
 	int			retval;
 };
@@ -1832,9 +1833,19 @@ bytes_unpend(struct req *req, struct bytes_tree *tree)
 	struct node_head work;
 	int retval;
 
+	CHECK_OBJ_NOTNULL(req, REQ_MAGIC);
+	CHECK_OBJ_NOTNULL(req->wrk, WORKER_MAGIC);
+
+	if (tree->unpend_owner != NULL)
+		return;
+
+	tree->unpend_owner = req->wrk;
+
 	do {
 		Lck_AssertHeld(&tree->tree_lock);
 
+		assert(tree->unpend_owner == req->wrk);
+
 		VSTAILQ_INIT(&work);
 
 		bytes_unpend_worklist(req, tree, &work);
@@ -1851,6 +1862,8 @@ bytes_unpend(struct req *req, struct bytes_tree *tree)
 		}
 		bytes_set_delivered_worklist(tree, &work);
 	} while (1);
+
+	tree->unpend_owner = NULL;
 }
 
 static int v_matchproto_(vdp_init_f)