mm: Add folio_young and folio_idle

Idle page tracking is handled through page_ext on 32-bit architectures.
Add folio equivalents for 32-bit and move all the page compatibility
parts to common code.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Howells <dhowells@redhat.com>