FreeBSD: Update dataset_kstats for zvols in dev mode

Previous commit added accounting for geom mode, but not for dev.
In geom mode we actually have GEOM statistics, while in dev mode
additional accounting actually makes more sense.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Closes #12097