Change the ISC_R_SUCCESS to DNS_RRL_RESULT_OK in dns_rrl()

Change the ISC_R_SUCCESS to DNS_RRL_RESULT_OK in dns_rrl()

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.