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

usr.sbin/bsdinstall/scripts/script

@@ -168,10 +168,10 @@ if [ -f $TMPDIR/bsdinstall-installscript-ab ]; then
 fi
 
 bsdinstall entropy
+bsdinstall umount
 if [ "$ZFSBOOT_DISKS" ]; then
 	zpool export $ZFSBOOT_POOL_NAME
 fi
-bsdinstall umount
 
 f_dprintf "Installation Completed at %s" "$( date )"