openvswitch: raise internal port TSO limit

Set the internal port TSO limit to GSO_MAX_SIZE (512 KB), as for
loopback and veth. This allows the admin/user to set a GSO limit up to
this value, to use BIG TCP for traffic routed through the internal port.

Only the capability limit changes: the IPv4 and IPv6 GSO and GRO limits
remain at 64 KiB until changed by the administrator.

Validation used a same-host test with a temporary OVS bridge, a veth pair,
and a network namespace:

  TCP client -> OVS internal -> OVS bridge -> veth -> TCP server

The veth endpoints were BIG TCP capable throughout the test, and only the
OVS internal port limits changed between 64 KiB and 524280 bytes. A single
iperf3 3.19 TCP flow, run separately over IPv4 and IPv6 with six
alternating runs per state (`-t 15 -O 5`, fixed CPU affinity and port
tuple), increased throughput by 18.1% for IPv4 and 34.7% for IPv6:

  protocol  BIG TCP off  BIG TCP on   CV off / on
  TCP/IPv4  65.567 Gbps  77.420 Gbps  1.12% / 0.37%
  TCP/IPv6  55.355 Gbps  74.568 Gbps  1.20% / 0.68%

Assisted-by: LLM
Signed-off-by: Wang Zhan <wang.zhan@smartx.com>
Reviewed-by: Ilya Maximets <i.maximets@ovn.org>
Reviewed-by: Eelco Chaudron <echaudro@redhat.com>
Link: https://patch.msgid.link/20260916065844.1380155-1-wang.zhan@smartx.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/openvswitch/vport-internal_dev.c

@@ -117,6 +117,8 @@ static void do_setup(struct net_device *netdev)
 	netdev->features |= NETIF_F_HW_VLAN_CTAG_TX | NETIF_F_HW_VLAN_STAG_TX;
 	netdev->hw_features = netdev->features;
 
+	netif_set_tso_max_size(netdev, GSO_MAX_SIZE);
+
 	eth_hw_addr_random(netdev);
 }