Revert "virtio: Restore mb() calls"

This reverts commit d99e725c26.

sys/dev/virtio/virtqueue.c

@@ -565,9 +565,6 @@ virtqueue_notify(struct virtqueue *vq)
 	/* Ensure updated avail->idx is visible to host. */
 	bus_dmamap_sync(vq->vq_ring_dmat, vq->vq_ring_mapp,
 	    BUS_DMASYNC_PREWRITE);
-#if defined(__i386__) || defined(__amd64__)
-	mb();
-#endif
 
 	if (vq_ring_must_notify_host(vq))
 		vq_ring_notify_host(vq);
@@ -963,9 +960,6 @@ vq_ring_enable_interrupt(struct virtqueue *vq, uint16_t ndesc)
 
 	bus_dmamap_sync(vq->vq_ring_dmat, vq->vq_ring_mapp,
 	    BUS_DMASYNC_PREWRITE);
-#if defined(__i386__) || defined(__amd64__)
-	mb();
-#endif
 
 	/*
 	 * Enough items may have already been consumed to meet our threshold