mm: convert huge_zero_page to huge_zero_folio

With all callers of is_huge_zero_page() converted, we can now switch the
huge_zero_page itself from being a compound page to a folio.

Link: https://lkml.kernel.org/r/20240326202833.523759-6-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>