workqueue: Fix NULL pointer dereference

When queue_work() is used in irq (not in task context), there is
a potential case that trigger NULL pointer dereference.
----------------------------------------------------------------
worker_thread()
|-spin_lock_irq()
|-process_one_work()
	|-worker->current_pwq = pwq
	|-spin_unlock_irq()
	|-worker->current_func(work)
	|-spin_lock_irq()
 	|-worker->current_pwq = NULL
|-spin_unlock_irq()

				//interrupt here
				|-irq_handler
					|-__queue_work()
						//assuming that the wq is draining
						|-is_chained_work(wq)
							|-current_wq_worker()
							//Here, 'current' is the interrupted worker!
								|-current->current_pwq is NULL here!
|-schedule()
----------------------------------------------------------------

Avoid it by checking for task context in current_wq_worker(), and
if not in task context, we shouldn't use the 'current' to check the
condition.

Reported-by: Xiaofei Tan <tanxiaofei@huawei.com>
Signed-off-by: Li Bin <huawei.libin@huawei.com>
Reviewed-by: Lai Jiangshan <jiangshanlai@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Fixes: 8d03ecfe ("workqueue: reimplement is_chained_work() using current_wq_worker()")
Cc: stable@vger.kernel.org # v3.9+