Ensure that soft updates are not enabled by default when using mdmfs(8)

When soft updates began being enabled by default that change carried
over to mdmfs(8) which does not want or need them. This fix ensures
that they are only enabled in mdmfs(8) when requested with the -U flag.

Reported by: Ivan Rozhuk
Tested by: Ivan Rozhuk
PR: 279308
MFC after: 1 week