ovpn: fix VPN TX bytes counting

ovpn: fix VPN TX bytes counting

In ovpn_net_xmit, after GSO segmentation and segment processing, the
first segment on the list is used to increment VPN TX statistics, which
fails to account for any subsequent segments in the chain.

Fix this by accumulating the length of every segment that successfully
passes skb_share_check into a tx_bytes variable. This ensures the peer
statistics accurately reflect the total data volume sent, regardless of
whether the original packet was segmented.

Fixes: 04ca14955f9a ("ovpn: store tunnel and transport statistics")
Signed-off-by: Ralf Lici <ralf@mandelbit.com>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Antonio Quartulli <antonio@openvpn.net>