mm: add virt_to_folio() and folio_address()

These two wrappers around their respective struct page variants will be
useful in the following patches.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Roman Gushchin <guro@fb.com>