mdmfs(8): use -o reserve with malloc-backed md(4)

Mentioned in mdconfig(8), malloc-backed md(4) can be unstable unless
required memory is allocated up front with -o reserve. Furthermore, panics
have been observed with md used in fstab on 12.0-RELEASE. Choose the stable
route and pass -o reserve.

Submitted by: Paul Vixie
MFC after: 1 week