Add const to nvlist functions to properly expose their real behavior

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Paul Dagnelie <pcd@delphix.com>
Closes #12728

module/zfs/vdev_label.c

@@ -669,7 +669,7 @@ vdev_config_generate(spa_t *spa, vdev_t *vd, boolean_t getstats,
 
 		if (idx) {
 			fnvlist_add_nvlist_array(nv, ZPOOL_CONFIG_CHILDREN,
-			    child, idx);
+			    (const nvlist_t * const *)child, idx);
 		}
 
 		for (c = 0; c < idx; c++)