btrfs: use folio_next_index() helper in extent_write_cache_pages

Simplify code pattern of 'folio->index + folio_nr_pages(folio)' by using
the existing helper folio_next_index().

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Minjie Du <duminjie@vivo.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>