virtio: remove overzealous BUG_ON.

{

	struct vring_virtqueue *vq = to_vvq(_vq);

	START_USE(vq);

	BUG_ON(vq->vring.avail->flags & VRING_AVAIL_F_NO_INTERRUPT);

	vq->vring.avail->flags |= VRING_AVAIL_F_NO_INTERRUPT;

	END_USE(vq);

}

static bool vring_enable_cb(struct virtqueue *_vq)

 *	Returns NULL or the "data" token handed to add_buf.

 * @disable_cb: disable callbacks

 *	vq: the struct virtqueue we're talking about.

 *	Note that this is not necessarily synchronous, hence unreliable and only

 *	useful as an optimization.

 * @enable_cb: restart callbacks after disable_cb.

 *	vq: the struct virtqueue we're talking about.

 *	This re-enables callbacks; it returns "false" if there are pending

 *	checking for more work, and enabling callbacks.

 *

 * Locking rules are straightforward: the driver is responsible for

 * locking.  No two operations may be invoked simultaneously.

 * locking.  No two operations may be invoked simultaneously, with the exception

 * of @disable_cb.

 *

 * All operations can be called in any context.

 */