async: make async_synchronize_full() more serializing

turns out that there are real problems with allowing async
tasks that are scheduled from async tasks to run after
the async_synchronize_full() returns.

This patch makes the _full more strict and a complete
synchronization. Later I might need to add back a lighter
form of synchronization for other uses.. but not right now.
Signed-off-by: NArjan van de Ven <arjan@linux.intel.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

kernel/async.c

@@ -206,7 +206,9 @@ EXPORT_SYMBOL_GPL(async_schedule_special);
 
 void async_synchronize_full(void)
 {
-	async_synchronize_cookie(next_cookie);
+	do {
+		async_synchronize_cookie(next_cookie);
+	} while (!list_empty(&async_running) || !list_empty(&async_pending));
 }
 EXPORT_SYMBOL_GPL(async_synchronize_full);