CSI node plugin fix for unstaging volumes

[beornf]

- What I did
While running a CSI volume plugin that supports staging, I created a new swarm service that initiated an attempt to unpublish a cluster volume. The node agent calls NodeUnpublishVolume which returns no errors then NodeUnstageVolume which also returns no errors. However, there was no call to the underlying plugin driver for NodeUnstageVolume.

- How I did it
Fixed the unpublish check to only return in the failing condition that the volume was not unpublished.

- How to test it
Create a cluster volume using CSI volume plugin then trigger a publish and unpublish by creating a swarm service and restarting the service.

- Description for the changelog

[beornf]

Similarly, this early return in NodeUnpublishVolume skips the log which would be helpful in debugging:

swarmkit/agent/csi/plugin/plugin.go

Lines 366 to 373 in ea1a7ce

	if v, ok := np.volumeMap[req.ID]; ok {
	v.publishedPath = ""
	v.isPublished = false
	return nil
	}
	log.G(ctx).Info("volume unpublished")
	return nil