workqueue: Make sure struct worker is accessible for wq_worker_comm()

The worker struct could already be freed when wq_worker_comm() tries
to access it for reporting.  This patch protects PF_WQ_WORKER
modifications with wq_pool_attach_mutex and makes wq_worker_comm()
test the flag before dereferencing worker from kthread_data(), which
ensures that it only dereferences when the worker struct is valid.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Lai Jiangshan <jiangshanlai@gmail.com>
Fixes: 6b59808b ("workqueue: Show the latest workqueue name in /proc/PID/{comm,stat,status}")