mm/hugetlb-cgroup: convert hugetlb_cgroup_css_offline() to work on folios

mm/hugetlb-cgroup: convert hugetlb_cgroup_css_offline() to work on folios

Let's convert hugetlb_cgroup_css_offline() and
hugetlb_cgroup_move_parent() to work on folios.  hugepage_activelist
contains folios, not pages.

While at it, rename page_hcg simply to hcg, removing most of the "page"
terminology.

This removes an unnecessary call to compound_head().

Link: https://lkml.kernel.org/r/20250113131611.2554758-6-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Sidhartha Kumar <sidhartha.kumar@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>