freezer: implement and use kthread_freezable_should_stop()

Writeback and thinkpad_acpi have been using thaw_process() to prevent
deadlock between the freezer and kthread_stop(); unfortunately, this
is inherently racy - nothing prevents freezing from happening between
thaw_process() and kthread_stop().

This patch implements kthread_freezable_should_stop() which enters
refrigerator if necessary but is guaranteed to return if
kthread_stop() is invoked.  Both thaw_process() users are converted to
use the new function.

Note that this deadlock condition exists for many of freezable
kthreads.  They need to be converted to use the new should_stop or
freezable workqueue.

Tested with synthetic test case.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Henrique de Moraes Holschuh <ibm-acpi@hmh.eng.br>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Oleg Nesterov <oleg@redhat.com>

fs/fs-writeback.c

@@ -947,7 +947,7 @@ int bdi_writeback_thread(void *data)
 
 	trace_writeback_thread_start(bdi);
 
-	while (!kthread_should_stop()) {
+	while (!kthread_freezable_should_stop(NULL)) {
 		/*
 		 * Remove own delayed wake-up timer, since we are already awake
 		 * and we'll take care of the preriodic write-back.
@@ -977,8 +977,6 @@ int bdi_writeback_thread(void *data)
 			 */
 			schedule();
 		}
-
-		try_to_freeze();
 	}
 
 	/* Flush any work that raced with us exiting */