mm/hugetlb_cgroup: identify the legacy using cgroup_subsys_on_dfl()

mm/hugetlb_cgroup: identify the legacy using cgroup_subsys_on_dfl()

Patch series "mm/hugetlb_cgroup: rework on cftypes", v3.

This patchset provides an intuitive view of the control files through
static templates of cftypes.  This improves the readability of the code.

This patch (of 3):

Currently the numa_stat file encodes 1 into .private using the micro
MEMFILE_PRIVATE() to identify the legacy.  Actually, we can use
cgroup_subsys_on_dfl() instead.  This is helpful to handle .private in the
static templates in the next patch.

Link: https://lkml.kernel.org/r/20240612092409.2027592-1-xiujianfeng@huawei.com
Link: https://lkml.kernel.org/r/20240612092409.2027592-2-xiujianfeng@huawei.com
Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Cc: Muchun Song <muchun.song@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>