Fixes rollback while unmounted.

Resolves https://github.com/openzfsonosx/zfs/issues/155

The note for dsl_dataset_rollback mentions
If owner != NULL:
- The existing dataset MUST be owned by the specified owner at entry
- Upon return, dataset will still be held by the same owner, whether we
  succeed or not.

So we need only be concerned if ds->ds_owner comes back NULL when
we were handed ddra->ddra_owner != NULL.

In particular, zfs_ioc_rollback will only call zfs_suspend_fs and
zfs_resume_fs when getzfsvfs(fsname, &zsb) succeeded.