Enforce PROP_ONETIME on zpool properties

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Closes #7661

module/zcommon/zpool_prop.c

@@ -171,6 +171,12 @@ zpool_prop_readonly(zpool_prop_t prop)
 	return (zpool_prop_table[prop].pd_attr == PROP_READONLY);
 }
 
+boolean_t
+zpool_prop_setonce(zpool_prop_t prop)
+{
+	return (zpool_prop_table[prop].pd_attr == PROP_ONETIME);
+}
+
 const char *
 zpool_prop_default_string(zpool_prop_t prop)
 {