mm/hugetlb_cgroup: fix -Wformat-truncation warning

mm/hugetlb_cgroup: fix -Wformat-truncation warning

A false-positive compile warnings with -Wformat-trucation was introduced
by commit 47179fe03588 ("mm/hugetlb_cgroup: prepare cftypes based on
template") on arch s390.  Suppress it by replacing snprintf() with
scnprintf().

mm/hugetlb_cgroup.c: In function 'hugetlb_cgroup_file_init':
mm/hugetlb_cgroup.c:829:44: warning: '%s' directive output may be truncated writing up to 1623 bytes into a region of size between 32 and 63 [-Wformat-truncation=]
  829 |   snprintf(cft->name, MAX_CFTYPE_NAME, "%s.%s", buf, tmpl->name);
      |                                            ^~

Link: https://lkml.kernel.org/r/20251222072359.3626182-1-xiujianfeng@huaweicloud.com
Fixes: 47179fe03588 ("mm/hugetlb_cgroup: prepare cftypes based on template")
Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202512212332.9lFRbgdS-lkp@intel.com/
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: "David Hildenbrand (Red Hat)" <david@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Oscar Salvador <osalvador@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>