Always use filepath.Join instead of path.Join

This patch cleans up pkg/util/mount/* and pkg/util/volume/* to always
use filepath.Join instead of path.Join. filepath.Join is preferred
because path.Join can have issues on Windows.

pkg/volume/csi/csi_plugin_test.go

@@ -756,7 +756,7 @@ func TestPluginNewUnmounter(t *testing.T) {
 	pv := makeTestPV("test-pv", 10, testDriver, testVol)
 
 	// save the data file to re-create client
-	dir := path.Join(getTargetPath(testPodUID, pv.ObjectMeta.Name, plug.host), "/mount")
+	dir := filepath.Join(getTargetPath(testPodUID, pv.ObjectMeta.Name, plug.host), "/mount")
 	if err := os.MkdirAll(dir, 0755); err != nil && !os.IsNotExist(err) {
 		t.Errorf("failed to create dir [%s]: %v", dir, err)
 	}