btrfs: convert relocate_one_page() to folios and rename

Convert page references to folios and call the respective folio
functions.  Since find_or_create_page() takes a mask argument, call
__filemap_get_folio() instead of filemap_grab_folio().

The patch assumes folio size is PAGE_SIZE, add a warning in case it's a
higher order that will be implemented in the future.

Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>