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.

Reported-by:  <syzbot+91dbdfecdd3287734d8e@syzkaller.appspotmail.com>
Cc: stable <stable@kernel.org>
Closes: https://lore.kernel.org/lkml/0000000000000ab25a061e1dfe9f@google.com/


Signed-off-by: Eli Billauer <eli.billauer@gmail.com>
Link: https://lore.kernel.org/r/20240801121126.60183-1-eli.billauer@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>