freezer: don't unnecessarily set PF_NOFREEZE explicitly

Some drivers set PF_NOFREEZE in their kthread functions which is
completely unnecessary and racy - some part of freezer code doesn't
consider cases where PF_NOFREEZE is set asynchronous to freezer
operations.

In general, there's no reason to allow setting PF_NOFREEZE explicitly.
Remove them and change the documentation to note that setting
PF_NOFREEZE directly isn't allowed.

-v2: Dropped change to twl4030-irq.c as it no longer uses PF_NOFREEZE.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: "Gustavo F. Padovan" <padovan@profusion.mobi>
Acked-by: Samuel Ortiz <sameo@linux.intel.com>
Cc: Marcel Holtmann <marcel@holtmann.org>
Cc: wwang <wei_wang@realsil.com.cn>

Documentation/power/freezing-of-tasks.txt

@@ -67,7 +67,7 @@ III. Which kernel threads are freezable?
 
 Kernel threads are not freezable by default.  However, a kernel thread may clear
 PF_NOFREEZE for itself by calling set_freezable() (the resetting of PF_NOFREEZE
-directly is strongly discouraged).  From this point it is regarded as freezable
+directly is not allowed).  From this point it is regarded as freezable
 and must call try_to_freeze() in a suitable place.
 
 IV. Why do we do that?

drivers/bluetooth/btmrvl_main.c

@@ -475,8 +475,6 @@ static int btmrvl_service_main_thread(void *data)
 
 	init_waitqueue_entry(&wait, current);
 
-	current->flags |= PF_NOFREEZE;
-
 	for (;;) {
 		add_wait_queue(&thread->wait_q, &wait);
 

drivers/mfd/twl6030-irq.c

@@ -138,8 +138,6 @@ static int twl6030_irq_thread(void *data)
 	static const unsigned max_i2c_errors = 100;
 	int ret;
 
-	current->flags |= PF_NOFREEZE;
-
 	while (!kthread_should_stop()) {
 		int i;
 		union {

drivers/staging/rts_pstor/rtsx.c

@@ -466,8 +466,6 @@ static int rtsx_control_thread(void *__dev)
 	struct rtsx_chip *chip = dev->chip;
 	struct Scsi_Host *host = rtsx_to_host(dev);
 
-	current->flags |= PF_NOFREEZE;
-
 	for (;;) {
 		if (wait_for_completion_interruptible(&dev->cmnd_ready))
 			break;