From kernel commit 34816d20f173a90389c8a7e641166d8ea9dce70a.
The L2TP_ATTR_UDP_ZERO_CSUM6_{TX,RX} attributes are introduced by 6b649feafe10b293f4bd5a74aca95faf625ae525, which is included in kernel-3.16. To support older kernel, let's import the header. Fixes #12300.