`chrdev` sample module triggers "free active" warning in debug mode

[TheSven73]

In debug mode, the chrdev sample module triggers a "free active" warning. Which probably points to a potential use-after-free issue.

This happens on every supported architecture, and is visible in the CI logs.

[   48.194412] ------------[ cut here ]------------
[   48.195307] ODEBUG: free active (active state 0) object type: timer_list hint: delayed_work_timer_fn+0x0/0x40
[   48.198373] WARNING: CPU: 0 PID: 38 at lib/debugobjects.c:508 debug_print_object+0xbf/0xe0
[   48.200944] Modules linked in: rust_chrdev(-) [last unloaded: rust_sync]

[alex]

fixed by #207