workqueue: Add workqueue_mayday and workqueue_rescued tracepoints

When a worker pool fails to create a new worker thread (e.g., typically
under severe memory pressure where memory reclaim paths depend on pending
work items), send_mayday() signals distress to the workqueue's rescuer
thread. The rescuer then takes over processing the pending work items via
assign_rescuer_work().

While pwq->stats[PWQ_STAT_MAYDAY] and pwq->stats[PWQ_STAT_RESCUED] track
these occurrences cumulatively, there is currently no event-driven
mechanism to observe exactly when mayday distress occurs or which work
items require rescue.

Add two new tracepoints namely workqueue_mayday and workqueue_rescued to
make distress and rescuer execution easily observable (e.g., via Ftrace
or eBPF).

Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

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