libct/cg/fs: add GetStats benchmark

On my CentOS 8 VM it shows:

> BenchmarkGetStats-8   	    8376	    625135 ns/op

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/cgroups/fs/fs_test.go

@@ -104,3 +104,35 @@ func TestTryDefaultCgroupRoot(t *testing.T) {
 		t.Errorf("tryDefaultCgroupRoot: want %q, got %q", exp, res)
 	}
 }
+
+func BenchmarkGetStats(b *testing.B) {
+	if cgroups.IsCgroup2UnifiedMode() {
+		b.Skip("cgroup v2 is not supported")
+	}
+
+	cg := &configs.Cgroup{
+		Path:      "/some/kind/of/a/path/here",
+		Resources: &configs.Resources{},
+	}
+	m := NewManager(cg, nil, false)
+	err := m.Apply(-1)
+	if err != nil {
+		b.Fatal(err)
+	}
+	defer func() {
+		_ = m.Destroy()
+	}()
+
+	var st *cgroups.Stats
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		st, err = m.GetStats()
+		if err != nil {
+			b.Fatal(err)
+		}
+	}
+	if st.CpuStats.CpuUsage.TotalUsage != 0 {
+		b.Fatalf("stats: %+v", st)
+	}
+}