Btrfs: fix race in worker_loop

Need to check kthread_should_stop after schedule_timeout() before calling
schedule(). This causes threads to sleep with potentially no one to wake them
up causing mount(2) to hang in btrfs_stop_workers waiting for threads to stop.
Signed-off-by: Amit Gud <gud@ksu.edu>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/async-thread.c

@@ -195,6 +195,9 @@ static int worker_loop(void *arg)
 				if (!list_empty(&worker->pending))
 					continue;
 
+				if (kthread_should_stop())
+					break;
+
 				/* still no more work?, sleep for real */
 				spin_lock_irq(&worker->lock);
 				set_current_state(TASK_INTERRUPTIBLE);
@@ -208,7 +211,8 @@ static int worker_loop(void *arg)
 				worker->working = 0;
 				spin_unlock_irq(&worker->lock);
 
-				schedule();
+				if (!kthread_should_stop())
+					schedule();
 			}
 			__set_current_state(TASK_RUNNING);
 		}