bsdinstall: mount is not needed for the ZFS install case

Because the datasets are already mounted by zfsboot, and the mount script
doesn't know anything about ZFS. Also do not execute the "umount" script for
ZFS for the same reasons.

Reviewed by: dteske, tsoome
Sponsored by: Citrix Systems R&D
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D10738