Fix panic on DilOS with kstat per dataset statistics

Account for ZFS_MAX_DATASET_NAME_LEN in kstat data size. This value
is ignored in the Linux kstat code but resolves the issue for other
platforms.

Reviewed-by: Serapheim Dimitropoulos <serapheim@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Igor Kozhukhov <igor@dilos.org>
Closes #9254
Closes #9151