mdmfs: Fix soft updates logic

Now that newfs(8) has a command-line argument to disable soft updates,
use that instead of running tunefs(8) after the fact to turn them off.

MFC after: 1 week
Sponsored by: Klara, Inc.
Sponsored by: NetApp, Inc.
Reviewed by: mckusick, imp
Differential Revision: https://reviews.freebsd.org/D54783