When the amt module is being removed, it calls flush_delayed_work() to exit
source_gc_wq. But it wouldn't be exited properly because the
amt_source_gc_work(), which is the callback function of source_gc_wq
internally calls mod_delayed_work() again.
So, amt_source_gc_work() would be called after the amt module is removed.
Therefore kernel panic would occur.
In order to avoid it, cancel_delayed_work() should be used instead of
flush_delayed_work().