workqueue: Fix NULL current_pwq deref in mem-reclaim helper

current_is_workqueue_mem_reclaim() uses current_wq_worker() to decide
whether %current is a workqueue worker and then checks the current
workqueue flags. However, current_wq_worker() only implies that %current
is a kworker; it does not imply that the worker is currently executing a
work item.

worker->current_pwq is set while process_one_work() runs the work
function and cleared afterwards. A kworker outside work-item execution can
therefore have PF_WQ_WORKER set with current_pwq cleared.

Guard the flag check with worker->current_pwq. Without a current_pwq, the
worker is not executing on a WQ_MEM_RECLAIM workqueue, so the helper
should return false.

Fixes: da729ddd4a1b ("NFS/localio: issue IO inline when not in a memory-reclaim context")
Signed-off-by: Pavankumar Kondeti <pavan.kondeti@oss.qualcomm.com>
Signed-off-by: Tejun Heo <tj@kernel.org>