btrfs: use common variable for fs_devices in btrfs_destroy_dev_replace_tgtdev

void btrfs_destroy_dev_replace_tgtdev(struct btrfs_fs_info *fs_info,

				      struct btrfs_device *tgtdev)

{

	struct btrfs_fs_devices *fs_devices = fs_info->fs_devices;

	WARN_ON(!tgtdev);

	mutex_lock(&fs_info->fs_devices->device_list_mutex);

	mutex_lock(&fs_devices->device_list_mutex);

	btrfs_sysfs_rm_device_link(fs_info->fs_devices, tgtdev);

	btrfs_sysfs_rm_device_link(fs_devices, tgtdev);

	if (tgtdev->bdev)

		fs_info->fs_devices->open_devices--;

		fs_devices->open_devices--;

	fs_info->fs_devices->num_devices--;

	fs_devices->num_devices--;

	btrfs_assign_next_active_device(fs_info, tgtdev, NULL);

	list_del_rcu(&tgtdev->dev_list);

	mutex_unlock(&fs_info->fs_devices->device_list_mutex);

	mutex_unlock(&fs_devices->device_list_mutex);

	/*

	 * The update_dev_time() with in btrfs_scratch_superblocks()