btrfs_assign_next_active_device
btrfs_assign_next_active_device(src_device, tgt_device);
btrfs_assign_next_active_device(device, NULL);
btrfs_assign_next_active_device(tgtdev, NULL);
btrfs_assign_next_active_device(fs_info->fs_devices->latest_dev,
void btrfs_assign_next_active_device(struct btrfs_device *device,