Read DCO traffic stats from the kernel

Read DCO traffic stats from the kernel

When DCO is active userspace doesn't see all of the traffic, so when we
access these stats we must update them.

Retrieve kernel statistics every time we access the
link_(read|write)_bytes values.

Introduce a dco_(read|write)_bytes so that we don't clobber the existing
statistics, which still count control packets, sent or received directly
through the socket.

Signed-off-by: Kristof Provost <kprovost@netgate.com>
Acked-by: Antonio Quartulli <antonio@openvpn.net>
Message-Id: <20221205164103.9190-2-kprovost@netgate.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg25618.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit ce2b459dabc29d071be28b8ddaa0512f8c8143ec)