[Code-Hex]

Virtio UDP_L4 GSO currently stops forwarding in vmnetsock. This PR adds segmentation into complete UDP datagrams for WithVirtioHeaderConversion and WithPeerVirtioHeaderNormalization.

The adapter emits datagrams with gso_size bytes of payload, except for a shorter final datagram. The adapter updates IP and UDP lengths, IPv4 IDs, and checksums. UDP segmentation has no connection state and does not require a TCP handshake. Dup and Close keep their existing contracts.

Scope

• Add IPv4 and IPv6 UDP_L4 segmentation to the existing bounded packet batches.
• Share the existing checksum arithmetic with TCP without changing its behavior.
• Add regression tests for datagram boundaries, checksum completion, invalid metadata, MTU limits, padding, input ownership, and batch write errors.

Limits

• GSO still requires vmnet.WithInterfaceTSO(true) because the adapter uses that setting for aggregate input and output buffers. Input must fit iface.MaxPacketSize().
• gso_size plus the IP and UDP headers must fit the interface MTU. Oversized values fail before output. Reducing gso_size would change UDP datagram boundaries.
• UDP_L4 requires NEEDS_CSUM with checksum metadata that matches the UDP header. Output has a complete checksum.
• UDP packets must have fixed IPv4 or IPv6 headers, without fragments or VLAN tags. IPv4 options, IPv6 extension headers, and jumbograms are unsupported.
• Legacy UFO and UDP RSC remain unsupported. The TCP-only GSO ECN modifier is invalid for UDP_L4. IP ECN bits are preserved.
• Local guest tests exercise UDP_L4 produced by Linux TAP with UDP_SEGMENT. They do not test USO negotiation by QEMU, VZ, or krunkit virtio NICs.

UDP_L4 segmentation follows Virtio 1.3 and the Linux distinction between USO and UFO.