By polling RTM_GETSTATS via netlink. RTM_GETSTATS is a relatively
efficient and targeted (single device) polling method available since
Linux v4.7.
The tundevstats "feature" can be extended to other platforms in the
future, and it's trivial to add new rtnl_link_stats64 counters on
Linux.
Updates tailscale/corp#38181
Signed-off-by: Jordan Whited <jordan@tailscale.com>
9c36a71a90