Fixes getNestedMountpoints grouping

Currently, getNestedMountpoints sorts using sort.Strings, which would
sort the following strings in this exact order:

/dir/nested, /dir/nested-vol, /dir/nested.vol, /dir/nested/double, /dir/nested2

Because of this, "nested/double" is returned as well, even though it shouldn't
have been. This issue is worse on Windows, where the path separator is typically
the backslash.

This commit addresses this issue by checking if a nested mount point has been
previously seen or not.

pkg/volume/util/nested_volumes_test.go

@@ -89,7 +89,7 @@ func TestGetNestedMountpoints(t *testing.T) {
 		{
 			name:     "Unsorted Nested Pod",
 			err:      false,
-			expected: sets.NewString("nested", "nested2"),
+			expected: sets.NewString("nested", "nested2", "nested-vol", "nested.vol"),
 			volname:  "vol1",
 			pod: v1.Pod{
 				ObjectMeta: metav1.ObjectMeta{
@@ -105,6 +105,9 @@ func TestGetNestedMountpoints(t *testing.T) {
 								{MountPath: "/dir/nested", Name: "vol2"},
 								{MountPath: "/ignore2", Name: "vol5"},
 								{MountPath: "/dir", Name: "vol1"},
+								{MountPath: "/dir/nested-vol", Name: "vol6"},
+								{MountPath: "/dir/nested.vol", Name: "vol7"},
+								{MountPath: "/dir/nested2/double", Name: "vol8"},
 								{MountPath: "/dir/nested2", Name: "vol3"},
 							},
 						},