io-wq: re-add io_wq_current_is_worker()

This reverts commit 8cdda87a, we now have several use csaes for this
helper. Reinstate it.
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io-wq.h

@@ -120,6 +120,10 @@ static inline void io_wq_worker_sleeping(struct task_struct *tsk)
 static inline void io_wq_worker_running(struct task_struct *tsk)
 {
 }
-#endif /* CONFIG_IO_WQ */
+#endif
 
-#endif /* INTERNAL_IO_WQ_H */
+static inline bool io_wq_current_is_worker(void)
+{
+	return in_task() && (current->flags & PF_IO_WORKER);
+}
+#endif