Is there some implicit locking of device methods?

[Bartosz Fabianowski]

> If you needs per-file private data for cdev, you would be better served
> by cdevpriv(9) KPI. Cloning is too hard to use correctly for such task.

Thanks, I just got that working. To help those going down a similar path 
in the future, I would like to note quickly that the following must be 
added to the cdevsw structure to ensure proper clean-up:

.d_flags = D_TRACKCLOSE

I just spent hours debugging panics until I realized only the last 
close() was triggering a call to my .d_close method.

- Bartosz