Btrfs: fix __btrfs_start_workers retval

__btrfs_start_workers returns 0 in case it raced with
btrfs_stop_workers and lost the race.  This is wrong because worker in
this case is not allowed to start and is in fact destroyed.  Return
-EINVAL instead.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

fs/btrfs/async-thread.c

@@ -495,6 +495,7 @@ static int __btrfs_start_workers(struct btrfs_workers *workers)
 	spin_lock_irq(&workers->lock);
 	if (workers->stopping) {
 		spin_unlock_irq(&workers->lock);
+		ret = -EINVAL;
 		goto fail_kthread;
 	}
 	list_add_tail(&worker->worker_list, &workers->idle_list);