Btrfs: async-thread: fix possible memory leak

When kthread_run() returns failure, this worker hasn't been
added to the list, so btrfs_stop_workers() won't free it.
Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/async-thread.c

@@ -187,6 +187,7 @@ int btrfs_start_workers(struct btrfs_workers *workers, int num_workers)
 		worker->task = kthread_run(worker_loop, worker, "btrfs");
 		worker->workers = workers;
 		if (IS_ERR(worker->task)) {
+			kfree(worker);
 			ret = PTR_ERR(worker->task);
 			goto fail;
 		}