char: xillybus: Don't destroy workqueue from work item running on it

In the Linux kernel, the following vulnerability has been resolved: char: xillybus: Don't destroy workqueue from work item running on it Triggered by a kref decrement, destroy_workqueue() may be called from within a work item for destroying its own workqueue. This illegal situation is averted by adding a module-global workqueue for exclusive use of the offending work item. Other work items continue to be queued on per-device workqueues to ensure performance.