bcachefs: comment bch_subvolume

	 * Snapshot subvolumes form a tree, separate from the snapshot nodes

	 * tree - if this subvolume is a snapshot, this is the ID of the

	 * subvolume it was created from:

	 *

	 * This is _not_ necessarily the subvolume of the directory containing

	 * this subvolume:

	 */

	__le32			parent;

	__le32			pad;