The patch at the end of this message fixes the following problems:

- cdev allocation moved outside st_dev_arr_lock, prevents the debugging
error messages reported by Mike and Mike
- cdev_unmap() added before cdev_del(), prevents oops (and kernel data
corruption) in case someone tries to use a device after removing the
module
- better error handling for failures in cdev allocation

As far as I am concerned, the patch is tested but testing by others might
be useful :-)