zfs_mount_all_mountpoints.ksh: cleanup_all should leave pool root mounted
zfs - Zettabyte File System
If pool root is not mounted, then zpool umount in next test will leave dataset mountpoint directory around and next zfs mount -a will fail with error:
cannot mount '/testpool': directory is not empty
Testing done: run zfs-tests, zfs_mount_encrypted is now PASS.