vhost: drop hard-coded num_buffers size

The 2 that we use for copy_to_iter comes from sizeof(u16),
it used to be that way before the iov iter update.
Fix it up, making it obvious the size of stack access
is right.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/vhost/net.c

@@ -621,7 +621,8 @@ static void handle_rx(struct vhost_net *net)
 
 		num_buffers = cpu_to_vhost16(vq, headcount);
 		if (likely(mergeable) &&
-		    copy_to_iter(&num_buffers, 2, &fixup) != 2) {
+		    copy_to_iter(&num_buffers, sizeof num_buffers,
+				 &fixup) != sizeof num_buffers) {
 			vq_err(vq, "Failed num_buffers write");
 			vhost_discard_vq_desc(vq, headcount);
 			break;