Documentation: kernel-parameters: add workqueue.panic_on_stall

The workqueue.panic_on_stall kernel parameter was added in commit
073107b3 ("workqueue: add cmdline parameter workqueue.panic_on_stall")
but not listed in the kernel-parameters doc. Add it there.

Signed-off-by: Sangmoon Kim <sangmoon.kim@samsung.com>
Signed-off-by: Tejun Heo <tj@kernel.org>