Add support for missing TUN/TAP parameters from Linux kernel

[vanytsvetkov]

Hey folks! This PR fixes TUN/TAP implementation to handle several kernel parameters we've been missing. No rocket science – just closing gaps with well-known features.

What's changed

1. New field in Tuntap struct

   • Added DisabledQueues to track inactive multiqueue slots (critical for live migration in VMs).

2. Now parsing kernel flags

   • IFLA_TUN_PI → sets TUNTAP_NO_PI (for "raw" packet mode)
   • IFLA_TUN_VNET_HDR → sets TUNTAP_VNET_HDR (QEMU/virtio needs this)
   • IFLA_TUN_MULTI_QUEUE support
   • Reads actual queue counts from IFLA_TUN_NUM_QUEUES/DISABLED_QUEUES

Why this is needed

• We were dropping important settings: Before this, flags like VNET_HDR or multiqueue configs got silently ignored.
• Kernel compatibility: These parameters exist since Linux 3.8 (2012!) but our code never acknowledged them.
• Real-world impact:
  • Cloud setups need DisabledQueues for VM migrations
  • MULTI_QUEUE matters for anyone using DPDK/VPP for fast packet processing

How I tested

• Ran on real 5.10+ kernels with:
  ✓ 4 active + 2 disabled queues
  ✓ VNET_HDR flag (matching QEMU's requirements)
  ✓ Legacy configs (verified old behavior stays intact)

Notes for review

• Zero breakage:
  • New fields start as 0 (backward-safe)
  • Older kernels without these attributes work normally
• Matches Linux tooling: Logic aligns with iproute2's implementation (see here) – we're not inventing wheels.

Signed-off-by: Ivan Tsvetkov [email protected]