Btrfs: fix use after free in btrfs_start_workers fail path

worker memory is already freed on one fail path in btrfs_start_workers,
but is still dereferenced. Switch the dereference and kfree.
Signed-off-by: NJiri Slaby <jirislaby@gmail.com>
Signed-off-by: NChris Mason <chris.mason@oracle.com>

fs/btrfs/async-thread.c

@@ -299,8 +299,8 @@ int btrfs_start_workers(struct btrfs_workers *workers, int num_workers)
 					   "btrfs-%s-%d", workers->name,
 					   workers->num_workers + i);
 		if (IS_ERR(worker->task)) {
-			kfree(worker);
 			ret = PTR_ERR(worker->task);
+			kfree(worker);
 			goto fail;
 		}