rc.initdiskless: Disable soft-updates in mdmfs (again)

Re-apply the -S switch to disable soft-updates in memory disks (commit
8b1292ac5219). This might be beneficial when tmpfs(5) is not present in
the kernel, as this can cause mdmfs(8)'s auto keyword to fallback to
using md(4).

PR: 85558
MFC after: 1 week
Reviewed by: imp, emaste, brooks
Differential Revision: https://reviews.freebsd.org/D43242