Use app.driver.Stat for registry health check

`app.driver.List` on `"/"` is very expensive if registry contains significant amount of images. And the result isn't used anyways.
In most (if not all) storage drivers, `Stat` has a cheaper implementation, so use it instead to achieve the same goal.

Signed-off-by: yixi zhang <yixi@memsql.com>

registry/handlers/app.go

@@ -345,7 +345,7 @@ func (app *App) RegisterHealthChecks(healthRegistries ...*health.Registry) {
 		}
 
 		storageDriverCheck := func() error {
-			_, err := app.driver.List(app, "/") // "/" should always exist
+			_, err := app.driver.Stat(app, "/") // "/" should always exist
 			return err                          // any error will be treated as failure
 		}