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
 		}
 

registry/storage/driver/base/base.go

@@ -137,7 +137,7 @@ func (base *Base) Stat(ctx context.Context, path string) (storagedriver.FileInfo
 	ctx, done := context.WithTrace(ctx)
 	defer done("%s.Stat(%q)", base.Name(), path)
 
-	if !storagedriver.PathRegexp.MatchString(path) {
+	if !storagedriver.PathRegexp.MatchString(path) && path != "/" {
 		return nil, storagedriver.InvalidPathError{Path: path, DriverName: base.StorageDriver.Name()}
 	}