makefs/zfs: Fill out dd_used_breakdown fields in DSL directories

This is required for the "used", "usedds" and "usedchild" dataset
properties to be displayed.

Approved by: re (gjb)
PR: 274613
Reported by: Mike Tancsa <mike@sentex.net>
MFC after: 1 week
Sponsored by: The FreeBSD Foundation

(cherry picked from commit eb5cbd793e31f9d656999033be54dac1a1107294)