ZFS ensure_filesystem call may race with `zfs list` and `zfs create`

[jordanhendricks]

The utility function ensure_filesystem in the illumos-utils ZFS code has the intention of creating a ZFS fileysystem, unless it already exists. It first performs a zfs list via dataset_exists, then a zfs create. This is inherently racy. The zfs create command has a -p flag that will give us the behavior we want here:

       -p  Creates all the non-existing parent datasets.  Datasets created in
           this manner are automatically mounted according to the mountpoint
           property inherited from their parent.  Any property specified on
           the command line using the -o option is ignored.  If the target
           filesystem already exists, the operation completes successfully.

We should switch to using zfs create -p here instead of listing the datasets first.

Per @smklein, this probably isn't a problem yet because we tend to have singletons controlling filesystem allocation. So probably not high priority for now.