workqueue: wq_watchdog_touch is always called with valid CPU

[ Upstream commit 18e24deb ]

Warn in the case it is called with cpu == -1. This does not appear
to happen anywhere.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>