Fix bsdinstall for root-on-zfs with MBR partitioning

Fix an error where vfs.root.mountfrom was not always set as required
when creating a bootpool. After the recent geliboot changes, it was only
set if the main pool was encrypted.

Also resolve an error where the bootpool was unmounted twice causing
bsdinstall to stop with an error message about the failed command.

Approved by: re (gjb)
Sponsored by: BSDCan Hacker Lounge