FreeBSD: Fix memory leaks in kstats

Don't handle (incorrectly) kmem_zalloc() failure. With KM_SLEEP,
will never return NULL.

Free the data allocated for non-virtual kstats when deleting the object.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #11767