workqueue: Rename pool->watchdog_ts to pool->last_progress_ts

workqueue: Rename pool->watchdog_ts to pool->last_progress_ts

The watchdog_ts name doesn't convey what the timestamp actually tracks.
This field tracks the last time a workqueue got progress.

Rename it to last_progress_ts to make it clear that it records when the
pool last made forward progress (started processing new work items).

No functional change.

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