net: remove unnecessary locking from RX buffer pools

[jstarks]

The RX buffer pools in virtio-net and GDMA/BNIC were wrapped in Arc<Mutex<>> (or Arc<Vec<Mutex<>>>), but they are only ever accessed from the single task that runs the queue. The locking was vestigial from an earlier design where the pool was shared. Removing it simplifies the code--no more Arc cloning, scratch buffers, or lock() calls on every buffer operation.

Performance is basically unchanged by this, which is not too surprising since the locks are uncontended and there is so much other lower-hanging fruit. But this change simplifies the code, so we should go ahead and take it anyway.

[Copilot]

Pull request overview

Removes vestigial locking/Arc indirection around RX buffer pools in virtio-net and GDMA/BNIC by making the pools task-local, simplifying buffer management and reducing per-packet overhead.

Changes:

• virtio-net: removes ActiveState.mem and uses the RX pool’s GuestMemory; replaces “collect ready IDs” allocation with an in-place fill_ready call.
• virtio-net buffers: replaces Arc<Vec<Mutex<...>>> with Vec<Option<...>> and adjusts APIs to require &mut self where mutation occurs.
• GDMA/BNIC: replaces Arc<Mutex<Slab<_>>> + scratch cloning with a task-local Slab<_> stored in GuestBuffers.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
vm/devices/virtio/virtio_net/src/lib.rs	Switches queue restart + TX header reads to use the RX pool as the single owner of GuestMemory and removes clone/lock-based ready collection.
vm/devices/virtio/virtio_net/src/buffers.rs	Makes the virtio RX work pool single-owner (no Arc<Mutex<...>>), adds fill_ready and exposes mem() accessor.
vm/devices/net/gdma/src/bnic.rs	Makes BNIC RX packet storage task-local (no Arc<Mutex<_>>) and removes scratch segment cloning.

[github-actions]

2 Petri tests failed (1 unstable)