GetMountRefs shouldn't error when file doesn'g exist in Windows and nsenter. Add unit test

pkg/util/mount/mount_linux.go

@@ -1007,6 +1007,8 @@ func (mounter *Mounter) SafeMakeDir(subdir string, base string, perm os.FileMode
 func (mounter *Mounter) GetMountRefs(pathname string) ([]string, error) {
 	if _, err := os.Stat(pathname); os.IsNotExist(err) {
 		return []string{}, nil
+	} else if err != nil {
+		return nil, err
 	}
 	realpath, err := filepath.EvalSymlinks(pathname)
 	if err != nil {

pkg/util/mount/mount_linux_test.go

@@ -110,6 +110,10 @@ func TestGetMountRefs(t *testing.T) {
 				"/var/lib/kubelet/plugins/kubernetes.io/gce-pd/mounts/gce-pd2",
 			},
 		},
+		{
+			"/var/fake/directory/that/doesnt/exist",
+			[]string{},
+		},
 	}
 
 	for i, test := range tests {

pkg/util/mount/mount_windows.go

@@ -459,6 +459,11 @@ func getAllParentLinks(path string) ([]string, error) {
 }
 
 func (mounter *Mounter) GetMountRefs(pathname string) ([]string, error) {
+	if _, err := os.Stat(normalizeWindowsPath(pathname)); os.IsNotExist(err) {
+		return []string{}, nil
+	} else if err != nil {
+		return nil, err
+	}
 	refs, err := getAllParentLinks(normalizeWindowsPath(pathname))
 	if err != nil {
 		return nil, err

pkg/util/mount/nsenter_mount.go

@@ -337,6 +337,13 @@ func (mounter *NsenterMounter) SafeMakeDir(subdir string, base string, perm os.F
 }
 
 func (mounter *NsenterMounter) GetMountRefs(pathname string) ([]string, error) {
+	exists, err := mounter.ExistsPath(pathname)
+	if err != nil {
+		return nil, err
+	}
+	if !exists {
+		return []string{}, nil
+	}
 	hostpath, err := mounter.ne.EvalSymlinks(pathname, true /* mustExist */)
 	if err != nil {
 		return nil, err