btrfs: page to folio conversion: prealloc_file_extent_cluster()

Convert usage of page to folio in prealloc_file_extent_cluster()

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>