workqueue: Add stall detector sample module

workqueue: Add stall detector sample module

Add a sample module under samples/workqueue/stall_detector/ that
reproduces a workqueue stall caused by PF_WQ_WORKER misuse.  The
module queues two work items on the same per-CPU pool, then clears
PF_WQ_WORKER and sleeps in wait_event_idle(), hiding from the
concurrency manager and stalling the second work item indefinitely.

This is useful for testing the workqueue watchdog stall diagnostics.

Signed-off-by: Breno Leitao <leitao@debian.org>
Acked-by: Song Liu <song@kernel.org>
Signed-off-by: Tejun Heo <tj@kernel.org>