When the circular bucket meta magic was not found, it is assumed that
the whole written state is in the old on-storage format. In that case,
the size of the circular bucket meta must not be subtracted from the
read length, or the (complete_len > len) check in the raw backend's
verify function will fail:

    state: Error, invalid data_len 16 in header, have data of len 24

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>