makefs: Fix space accounting for the root DSL directory

Space used by the MOS is summed after all on-disk data structures are
finalized, so the MOS DSL directory represents a special case when
counting bytes used for each directory. The DSL layer failed to update
the MOS DSL directory's parent, the root directory, accordingly, so the
root directory reported less space used than was used in reality. Be
careful to update the root directory after the MOS is finalized.

PR: 265849
Sponsored by: The FreeBSD Foundation