checkpoint/restore: fix cgroupv2 handling

In case of cgroupv2 unified hierarchy, the /sys/fs/cgroup mount
is the real mount with fstype of cgroup2 (rather than a set of
external bind mounts like for cgroupv1).

So, we should not add it to the list of "external bind mounts"
on both checkpoint and restore.

Without this fix, checkpoint integration tests fail on cgroup v2.

Also, same is true for cgroup v1 + cgroupns.

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

libcontainer/rootfs_linux.go

@@ -244,7 +244,7 @@ func mountCgroupV1(m *configs.Mount, rootfs, mountLabel string, enableCgroupns b
 			}
 			cgroupmount := &configs.Mount{
 				Source:      "cgroup",
-				Device:      "cgroup",
+				Device:      "cgroup", // this is actually fstype
 				Destination: subsystemPath,
 				Flags:       flags,
 				Data:        filepath.Base(subsystemPath),