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

module/zfs/dataset_kstats.c

@@ -135,6 +135,7 @@ dataset_kstats_create(dataset_kstats_t *dk, objset_t *objset)
 	kstat->ks_data = dk_kstats;
 	kstat->ks_update = dataset_kstats_update;
 	kstat->ks_private = dk;
+	kstat->ks_data_size += ZFS_MAX_DATASET_NAME_LEN;
 
 	kstat_install(kstat);
 	dk->dk_kstats = kstat;