virtio: Fix polling in virtqueue_dequeue()

The access of vq->vq_ring.used->idx needs to be volatile-qualified,
otherwise the compiler may optimize virtqueue_poll() into an infinite
loop if there is no data available upon the first poll.

Prior to commit ad17789 this wasn't a problem since an external
function call after each poll inhibited the optimization.

PR:		289930
MFC after:	3 days
Sponsored by:	Klara, Inc.
Fixes:		ad17789 ("virtio: Remove the unused poll method")

Author: markjdb

sys/dev/virtio/virtqueue.c

@@ -580,7 +580,8 @@ virtqueue_dequeue(struct virtqueue *vq, uint32_t *len)
 	void *cookie;
 	uint16_t used_idx, desc_idx;
 
-	if (vq->vq_used_cons_idx == vq_htog16(vq, vq->vq_ring.used->idx))
+	if (vq->vq_used_cons_idx ==
+	    vq_htog16(vq, atomic_load_16(&vq->vq_ring.used->idx)))
 		return (NULL);
 
 	used_idx = vq->vq_used_cons_idx++ & (vq->vq_nentries - 1);