Rename parent and data

'parent' function is confusing with parent cgroup, it's actually
parent path, so rename it to parentPath.

The name 'data' is too common to be identified, rename it to cgroupData
which is exactly what it is.

Signed-off-by: Qiang Huang <h.huangqiang@huawei.com>

libcontainer/cgroups/fs/util_test.go

@@ -17,8 +17,8 @@ import (
 )
 
 type cgroupTestUtil struct {
-	// data to use in tests.
-	CgroupData *data
+	// cgroup data to use in tests.
+	CgroupData *cgroupData
 
 	// Path to the mock cgroup directory.
 	CgroupPath string
@@ -30,7 +30,7 @@ type cgroupTestUtil struct {
 
 // Creates a new test util for the specified subsystem
 func NewCgroupTestUtil(subsystem string, t *testing.T) *cgroupTestUtil {
-	d := &data{
+	d := &cgroupData{
 		c: &configs.Cgroup{},
 	}
 	tempDir, err := ioutil.TempDir("", "cgroup_test")