mm/migrate: Use a folio in alloc_migration_target()

This removes an assumption that a large folio is HPAGE_PMD_ORDER
as well as letting us remove the call to prep_transhuge_page()
and a few hidden calls to compound_head().

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