respect max_restarts and fix c00028.vtc

Fixes #2949

bin/varnishd/cache/cache_req_fsm.c

@@ -272,6 +272,10 @@ cnt_synth(struct worker *wrk, struct req *req)
 		return (REQ_FSM_DONE);
 	}
 
+	if (wrk->handling == VCL_RET_RESTART &&
+	    req->restarts > cache_param->max_restarts)
+		wrk->handling = VCL_RET_DELIVER;
+
 	if (wrk->handling == VCL_RET_RESTART) {
 		/*
 		 * XXX: Should we reset req->doclose = SC_VCL_FAILURE

bin/varnishtest/tests/c00028.vtc

@@ -8,7 +8,11 @@ varnish v1 -vcl {
 	sub vcl_backend_fetch {
 	    set bereq.backend = bad;
 	}
+	sub vcl_backend_error {
+	    return (abandon);
+	}
 	sub vcl_synth {
+	    set resp.http.restarts = req.restarts;
 	    return (restart);
 	}
  } -start
@@ -17,4 +21,5 @@ client c1 {
 	txreq -url "/"
 	rxresp
 	expect resp.status == 503
+	expect resp.http.restarts == 5
 } -run