linux: Backport support for BIG TCP GSO on WireGuard

linux: Backport support for BIG TCP GSO on WireGuard

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: Michael Tremer <michael.tremer@ipfire.org>