btrfs: switch grab_extent_buffer() to folios

Use the folio API, remove page_folio/folio_page conversions.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>