btrfs_scan_one_device
device = btrfs_scan_one_device(fc->source, true);
device = btrfs_scan_one_device(vol->name, false);
device = btrfs_scan_one_device(vol->name, false);
device = btrfs_scan_one_device(param->string, false);
struct btrfs_device *btrfs_scan_one_device(const char *path, bool mount_arg_dev);