In probes treat a poll timeout as a recv error

bin/varnishd/cache/cache_backend_probe.c

@@ -271,6 +271,8 @@ vbp_poke(struct vbp_target *vt)
 		if (tmo > 0)
 			i = poll(pfd, 1, tmo);
 		if (i == 0 || tmo <= 0) {
+			if (i == 0)
+				vt->err_recv |= 1;
 			VTCP_close(&s);
 			return;
 		}