message: rafs: fix up some RAFS v6 description and validator
  It's possible that self.s_meta_blkaddr == 0 and
  self.s_xattr_blkaddr != 0 for shared xattr use as RAFS v6 images.
diff: @@ -58,7 +58,7 @@ const EROFS_CHUNK_FORMAT_SIZE_MASK: u16 = 0x001F;
  /// Checksum of superblock, compatible with EROFS versions prior to Linux kernel 5.5.
  #[allow(dead_code)]
  const EROFS_FEATURE_COMPAT_SB_CHKSUM: u32 = 0x0000_0001;
  -/// Rafs v6 specific metadata, compatible with EROFS versions prior to Linux kernel 5.16.
  +/// Rafs v6 specific metadata, compatible with EROFS versions since Linux kernel 5.16.
  const EROFS_FEATURE_COMPAT_RAFS_V6: u32 = 0x4000_0000;
  /// Chunked inode, incompatible with EROFS versions prior to Linux kernel 5.15.
  const EROFS_FEATURE_INCOMPAT_CHUNKED_FILE: u32 = 0x0000_0004;
  @@ -146,14 +146,6 @@ impl RafsV6SuperBlock {
  Err(einval!("invalid blocks in Rafsv6 superblock"))
  } else if u16::from_le(self.s_u) != 0 {
  Err(einval!("invalid union field in Rafsv6 superblock"))
  - } else if u32::from_le(self.s_meta_blkaddr) == 0 {
  - Err(einval!(
  - "invalid metadata block address in Rafsv6 superblock"
  - ))
  - } else if u32::from_le(self.s_xattr_blkaddr) != 0 {
  - Err(einval!(
  - "invalid extended attribute block address in Rafsv6 superblock"
  - ))
  } else if u64::from_le(self.s_build_time) != 0 || u32::from_le(self.s_build_time_nsec) != 0
  {
  Err(einval!("invalid build time in Rafsv6 superblock"))
  @@ -167,7 +159,7 @@ impl RafsV6SuperBlock {
  != EROFS_FEATURE_COMPAT_RAFS_V6
  {
  Err(einval!(
  - "invalid incompatible feature bits in Rafsv6 superblock"
  + "invalid compatible feature bits in Rafsv6 superblock"
  ))
  } else {
  Ok(())
  