mm: Non-pmd-mappable, large folios for folio_add_new_anon_rmap()

In preparation for FLEXIBLE_THP support, improve
folio_add_new_anon_rmap() to allow a non-pmd-mappable, large folio to be
passed to it. In this case, all contained pages are accounted using the
"small" pages scheme.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>