vhost: replace % with & on data path

We know vring num is a power of 2, so use &
to mask the high bits.
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

drivers/vhost/vhost.c

@@ -1369,7 +1369,7 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
 	/* Grab the next descriptor number they're advertising, and increment
 	 * the index we've seen. */
 	if (unlikely(__get_user(ring_head,
-				&vq->avail->ring[last_avail_idx % vq->num]))) {
+				&vq->avail->ring[last_avail_idx & (vq->num - 1)]))) {
 		vq_err(vq, "Failed to read head: idx %d address %p\n",
 		       last_avail_idx,
 		       &vq->avail->ring[last_avail_idx % vq->num]);
@@ -1489,7 +1489,7 @@ static int __vhost_add_used_n(struct vhost_virtqueue *vq,
 	u16 old, new;
 	int start;
 
-	start = vq->last_used_idx % vq->num;
+	start = vq->last_used_idx & (vq->num - 1);
 	used = vq->used->ring + start;
 	if (count == 1) {
 		if (__put_user(heads[0].id, &used->id)) {
@@ -1531,7 +1531,7 @@ int vhost_add_used_n(struct vhost_virtqueue *vq, struct vring_used_elem *heads,
 {
 	int start, n, r;
 
-	start = vq->last_used_idx % vq->num;
+	start = vq->last_used_idx & (vq->num - 1);
 	n = vq->num - start;
 	if (n < count) {
 		r = __vhost_add_used_n(vq, heads, n);