ovpn: store tunnel and transport statistics

Byte/packet counters for in-tunnel and transport streams
are now initialized and updated as needed.

To be exported via netlink.

Signed-off-by: Antonio Quartulli <antonio@openvpn.net>
Link: https://patch.msgid.link/20250415-b4-ovpn-v26-10-577f6097b964@openvpn.net


Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>