cleanup implicit rollback for return(vcl(...))

VCC generates a call to VPI_vcl_select() before returning to the fsm. We
basically conducted half a rollback in VPI_vcl_slect and a rull rollback
in the fsm.

De-duplicate this dance and assert in the fsm that our request looks
rolled back.

bin/varnishd/cache/cache_req_fsm.c

@@ -879,7 +879,8 @@ cnt_recv(struct worker *wrk, struct req *req)
 
 	VCL_recv_method(req->vcl, wrk, req, NULL, NULL);
 	if (wrk->handling == VCL_RET_VCL && req->restarts == 0) {
-		Req_Rollback(req);
+		// Req_Rollback has happened in VPI_vcl_select
+		assert(WS_Snapshot(req->ws) == req->ws_req);
 		cnt_recv_prep(req, ci);
 		VCL_recv_method(req->vcl, wrk, req, NULL, NULL);
 	}

bin/varnishd/cache/cache_vpi.c

@@ -99,12 +99,8 @@ VPI_vcl_select(VRT_CTX, VCL_VCL vcl)
 	if (! IS_TOPREQ(req))
 		assert(req->vcl == req->top->vcl0);
 
-	/* XXX VCL_Task* are somewhat duplicated to those in Req_Rollback called
-	 * from FSM for VCL_RET_VCL. Keeping them here to ensure there are no
-	 * tasks during calls to VCL_Rel / vcl_get
-	 */
-	VCL_TaskLeave(req->top->privs);
-	VCL_TaskLeave(req->privs);
+	Req_Rollback(req);
+
 	if (IS_TOPREQ(req)) {
 		AN(req->top);
 		AZ(req->top->vcl0);
@@ -116,6 +112,4 @@ VPI_vcl_select(VRT_CTX, VCL_VCL vcl)
 	vcl_get(&req->vcl, vcl);
 	VSLb(ctx->req->vsl, SLT_VCL_use, "%s via %s",
 	    req->vcl->loaded_name, vcl->loaded_name);
-	VCL_TaskEnter(req->privs);
-	VCL_TaskEnter(req->top->privs);
 }