net/netcheck: fix race condition initializting RegionLatency maps.

Under some conditions, code would try to look things up in the maps
before the first call to updateLatency. I don't see any reason to delay
initialization of the maps, so let's just init them right away when
creating the Report instance.

Signed-off-by: Avery Pennarun <apenwarr@tailscale.com>

net/netcheck/netcheck_test.go

@@ -96,7 +96,7 @@ func TestWorksWhenUDPBlocked(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	want := new(Report)
+	want := newReport()
 
 	if !reflect.DeepEqual(r, want) {
 		t.Errorf("mismatch\n got: %+v\nwant: %+v\n", r, want)