makefs/zfs: Update dd_used_breakdown after writing the MOS

Approved by: re (gjb)
Fixes: 96092bf9108c ("makefs/zfs: Fill out dd_used_breakdown fields in DSL directories")
Sponsored by: The FreeBSD Foundation

(cherry picked from commit c9c029507f80b31fb67b1b545f3062c7266d5f9e)