Add a protocol spec for tcp_metrics, so that it's accessible via YNL.
Useful at the very least for testing fixes.

In this episode of "10,000 ways to complicate netlink" the metric
nest has defines which are off by 1. iproute2 does:

        struct rtattr *m[TCP_METRIC_MAX + 1 + 1];

        parse_rtattr_nested(m, TCP_METRIC_MAX + 1, a);

        for (i = 0; i < TCP_METRIC_MAX + 1; i++) {
                // ...
                attr = m[i + 1];

This is too weird to support in YNL, add a new set of defines
with _correct_ values to the official kernel header.
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>