hugetlb: convert alloc_buddy_hugetlb_folio to use a folio

hugetlb: convert alloc_buddy_hugetlb_folio to use a folio

While this function returned a folio, it was still using __alloc_pages()
and __free_pages().  Use __folio_alloc() and put_folio() instead.  This
actually removes a call to compound_head(), but more importantly, it
prepares us for the move to memdescs.

Link: https://lkml.kernel.org/r/20240402200656.913841-1-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Sidhartha Kumar <sidhartha.kumar@oracle.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Reviewed-by: Muchun Song <muchun.song@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>