btrfs: document mandatory order of bio in btrfs_io_bio

	u8 *csum_allocated;

	btrfs_io_bio_end_io_t *end_io;

	struct bvec_iter iter;

	/*

	 * This member must come last, bio_alloc_bioset will allocate enough

	 * bytes for entire btrfs_io_bio but relies on bio being last.

	 */

	struct bio bio;

};