Re: [RFC RESEND 1/3] virtio: add virtqueue_get_last_used_idx() helper

From: Longjun Tang <redacted>

Export a read-only accessor for the last consumed used ring index.
It's useful for drivers that need to detect whether the device has
produced buffers that have not been consumed yet, by passing the
returned value to virtqueue_poll().

The split ring now writes the field with WRITE_ONCE() (the packed
ring already did) to document that the new reader can run concurrently
with the poll path.

Assisted-by: Kilo:deepseek-v4-pro
Signed-off-by: Longjun Tang <redacted>

Sashiko has reviewed this patch and found no issues. It looks great!

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