message: rafs/v6: public some fields of RafsV6InodeExt
  So nydusd at runtime can access those info from disk inode.
diff: @@ -568,28 +568,29 @@ impl RafsStore for RafsV6InodeCompact {
  #[derive(Clone, Copy, Default, Debug)]
  pub struct RafsV6InodeExtended {
  /// Layout format for of the inode.
  - i_format: u16,
  + pub i_format: u16,
  /// TODO: doc
  - i_xattr_icount: u16,
  + pub i_xattr_icount: u16,
  /// Protection mode.
  - i_mode: u16,
  + pub i_mode: u16,
  i_reserved: u16,
  /// Size of the file content.
  - i_size: u64,
  - /// A `u32` union: raw_blkaddr or rdev or rafs_v6_inode_chunk_info
  - i_u: u32,
  + pub i_size: u64,
  + /// A `u32` union: raw_blkaddr or `rdev` or `rafs_v6_inode_chunk_info`
  + /// TODO: Use this field like C union style.
  + pub i_u: u32,
  /// Inode number.
  - i_ino: u32,
  + pub i_ino: u32,
  /// User ID of owner.
  - i_uid: u32,
  + pub i_uid: u32,
  /// Group ID of owner
  - i_gid: u32,
  - /// Time of last modification.
  - i_mtime: u64,
  - /// Time of last modification.
  - i_mtime_nsec: u32,
  - /// Number of hard links.
  - i_nlink: u32,
  + pub i_gid: u32,
  + /// Time of last modification - second part.
  + pub i_mtime: u64,
  + /// Time of last modification - nanoseconds part.
  + pub i_mtime_nsec: u32,
  + /// Number of links.
  + pub i_nlink: u32,
  i_reserved2: [u8; 16],
  }
  