ensure nexus reqs from tree have the esi_level == 0 worker

We pass on reqs from esi subrequests to the top request for delivery.
Doing so we need to give them the top requests's worker such that VDPs
requiring it be happy.

src/node.c

@@ -835,8 +835,17 @@ worklist_push(struct req *req, struct bytes_tree *tree,
 	node = VSTAILQ_FIRST(work);
 	AN(node);
 	while (node) {
+		parent = node->parent;
 		next = VSTAILQ_NEXT(node, unpend);
 
+		if (parent != NULL) {
+			assert (parent->type == T_NEXUS);
+			if (parent->nexus.req->wrk == NULL)
+				parent->nexus.req->wrk = req->wrk;
+			else
+				assert (parent->nexus.req->wrk == req->wrk);
+		}
+
 		// not using assert_node: it needs locking
 		assert(node->state == ST_UNPENDING);
 
@@ -861,7 +870,6 @@ worklist_push(struct req *req, struct bytes_tree *tree,
 			break;
 
 		if (VSTAILQ_NEXT(node, sibling) == NULL) {
-			parent = node->parent;
 			if (parent->parent == NULL)
 				(void) 0;
 			else if (parent->state < ST_CLOSED)