ptdesc: remove references to folios from __pagetable_ctor() and pagetable_dtor()

ptdesc: remove references to folios from __pagetable_ctor() and pagetable_dtor()

In preparation for splitting struct ptdesc from struct page and struct
folio, remove mentions of struct folio from these functions.  Introduce
ptdesc_nr_pages() to avoid using lruvec_stat_add/sub_folio()

Link: https://lkml.kernel.org/r/20250908171104.2409217-3-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>