Address inconsistencies in checking added RRsets

loading_addrdataset() rejects SOA RRsets which are not at top of zone.
addrdataset() should similarly reject such RRsets.

lib/dns/rbtdb.c

@@ -6798,6 +6798,13 @@ addrdataset(dns_db_t *db, dns_dbnode_t *node, dns_dbversion_t *version,
 	INSIST(rbtversion == NULL || rbtversion->rbtdb == rbtdb);
 
 	if (rbtdb->common.methods == &zone_methods) {
+		/*
+		 * SOA records are only allowed at top of zone.
+		 */
+		if (rdataset->type == dns_rdatatype_soa &&
+		    node != rbtdb->origin_node) {
+			return (DNS_R_NOTZONETOP);
+		}
 		RWLOCK(&rbtdb->tree_lock, isc_rwlocktype_read);
 		REQUIRE(((rbtnode->nsec == DNS_RBT_NSEC_NSEC3 &&
 			  (rdataset->type == dns_rdatatype_nsec3 ||