TestCgroupDriverSystemdMemoryLimit: fix failure

The test was failing:

  --- FAIL: TestCgroupDriverSystemdMemoryLimit (1.76s)
      cgroupdriver_systemd_test.go:54: assertion failed: 67108864 (s.HostConfig.Memory int64) != 67108864 (mem int)

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

integration/system/cgroupdriver_systemd_test.go

@@ -38,7 +38,7 @@ func TestCgroupDriverSystemdMemoryLimit(t *testing.T) {
 	d.StartWithBusybox(t, "--exec-opt", "native.cgroupdriver=systemd", "--iptables=false")
 	defer d.Stop(t)
 
-	const mem = 64 * 1024 * 1024 // 64 MB
+	const mem = int64(64 * 1024 * 1024) // 64 MB
 
 	ctx := context.Background()
 	ctrID := container.Create(ctx, t, c, func(ctr *container.TestContainerConfig) {