wireguard: device: support big tcp GSO

Advertise GSO_MAX_SIZE as TSO max size in order support BIG TCP for wireguard.
This helps to improve wireguard performance a bit when enabled as it allows
wireguard to aggregate larger skbs in wg_packet_consume_data_done() via
napi_gro_receive(), but also allows the stack to build larger skbs on xmit
where the driver then segments them before encryption inside wg_xmit().
We've seen a 15% improvement in TCP stream performance.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Link: https://patch.msgid.link/20241117212030.629159-5-Jason@zx2c4.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: borkmann

drivers/net/wireguard/device.c

@@ -302,6 +302,8 @@ static void wg_setup(struct net_device *dev)
 	/* We need to keep the dst around in case of icmp replies. */
 	netif_keep_dst(dev);
 
+	netif_set_tso_max_size(dev, GSO_MAX_SIZE);
+
 	wg->dev = dev;
 }