kernel: freezer should treat PF_IO_WORKER like PF_KTHREAD for freezing

Don't send fake signals to PF_IO_WORKER threads, they don't accept
signals. Just treat them like kthreads in this regard, all they need
is a wakeup as no forced kernel/user transition is needed.
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

kernel/freezer.c

@@ -134,7 +134,7 @@ bool freeze_task(struct task_struct *p)
 		return false;
 	}
 
-	if (!(p->flags & PF_KTHREAD))
+	if (!(p->flags & (PF_KTHREAD | PF_IO_WORKER)))
 		fake_signal_wake_up(p);
 	else
 		wake_up_state(p, TASK_INTERRUPTIBLE);