message: rafs: drop the uuid field
  By design, uuid and blob_id in the same union. uuid is not directly
  used by RAFS v6. Drop it instead.
diff: @@ -818,11 +818,9 @@ impl RafsStore for RafsV6InodeChunkAddr {
  #[repr(C)]
  #[derive(Clone, Copy, Debug)]
  pub struct RafsV6Device {
  - /// UUID for blob device.
  - uuid: [u8; 16],
  /// Blob id of sha256.
  blob_id: [u8; 32],
  - reserved1: [u8; 16],
  + reserved1: [u8; 32],
  /// Number of blocks on the device.
  blocks: u32,
  /// Mapping start address.
  @@ -833,9 +831,8 @@ pub struct RafsV6Device {
  impl Default for RafsV6Device {
  fn default() -> Self {
  Self {
  - uuid: [0u8; 16],
  blob_id: [0u8; 32],
  - reserved1: [0u8; 16],
  + reserved1: [0u8; 32],
  blocks: u32::to_le(0),
  mapped_blkaddr: u32::to_le(0),
  reserved2: [0u8; 56],
  