virtio: Avoid possible kernel panic if DEBUG is enabled.

The virtqueue_add() calls START_USE() upon entry. The virtqueue_kick() is
called if vq->num_added == (1 << 16) - 1 before calling END_USE().
The virtqueue_kick_prepare() called via virtqueue_kick() calls START_USE()
upon entry, and will call panic() if DEBUG is enabled.
Move this virtqueue_kick() call to after END_USE() call.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

drivers/virtio/virtio_ring.c

@@ -244,14 +244,14 @@ static inline int virtqueue_add(struct virtqueue *_vq,
 	vq->vring.avail->idx = cpu_to_virtio16(_vq->vdev, virtio16_to_cpu(_vq->vdev, vq->vring.avail->idx) + 1);
 	vq->num_added++;
 
+	pr_debug("Added buffer head %i to %p\n", head, vq);
+	END_USE(vq);
+
 	/* This is very unlikely, but theoretically possible.  Kick
 	 * just in case. */
 	if (unlikely(vq->num_added == (1 << 16) - 1))
 		virtqueue_kick(_vq);
 
-	pr_debug("Added buffer head %i to %p\n", head, vq);
-	END_USE(vq);
-
 	return 0;
 }