space-opt-migration: Small fixes

[robin-nitrokey]

Some small fixes for #55. They should eventually be squashed into the parent commit but for transparency during the review, I wanted to keep and merge them separately.

• We set total_credentials to Some(num_rks) just before accessing it, so we don’t need to go through the option.
• prev_filename is always set to Some so we can remove the Option.
• rp_dir is now always RK_DIR so we don’t need to cache it.