+ /* If a device has the given size, and the data_offset has been
+ * requested - work out how much space is available for data.
+ * This involves adjusting for reserved space (e.g. bitmaps)
+ * and for any rounding.
+ * 'mdadm' only calls this for existing arrays where a possible
+ * spare is being added. However some super-handlers call it
+ * internally from validate_geometry when creating an array.
+ */