There's value mismatch between the return type of dns_rrl() that's
dns_rrl_result_t and ISC_R_SUCCESS which belongs to isc_result_t. This
works incidentally, because DNS_RRL_RESULT_OK == ISC_R_SUCCESS.
This would break when we change isc_result_t to be static enum in
consecutive commit. Change the value to match the type.
4d85040df5