mm/mempolicy: Use vma_alloc_folio() in new_page()

Simplify new_page() by unifying the THP and base page cases, and
handle orders other than 0 and HPAGE_PMD_ORDER correctly.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>