bsdinstall/script: umount before zpool export

When running zpool export first, boot/efi and dev is still mounted so
zpool export fails. By running bsdinstall umount first the pool can be
cleanly exported.

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D35114
Sponsored by: Beckhoff Automation GmbH & Co. KG
MFC After: 3 days

(cherry picked from commit 450b4ac23c75e2dde996c96049c0887864e22d09)