mm/migrate: Use a folio in migrate_misplaced_transhuge_page()

Unify alloc_misplaced_dst_page() and alloc_misplaced_dst_page_thp().
Removes an assumption that compound pages are HPAGE_PMD_ORDER.

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