message: rafs/v6: better to count xattr name prefix in bytes
  String.len() may be bigger than bytes.
diff: @@ -1460,27 +1460,27 @@ lazy_static! {
  RafsV6XattrPrefix::new(
  XATTR_USER_PREFIX,
  EROFS_XATTR_INDEX_USER,
  - XATTR_USER_PREFIX.len()
  + XATTR_USER_PREFIX.as_bytes().len()
  ),
  RafsV6XattrPrefix::new(
  XATTR_NAME_POSIX_ACL_ACCESS,
  EROFS_XATTR_INDEX_POSIX_ACL_ACCESS,
  - XATTR_NAME_POSIX_ACL_ACCESS.len()
  + XATTR_NAME_POSIX_ACL_ACCESS.as_bytes().len()
  ),
  RafsV6XattrPrefix::new(
  XATTR_NAME_POSIX_ACL_DEFAULT,
  EROFS_XATTR_INDEX_POSIX_ACL_DEFAULT,
  - XATTR_NAME_POSIX_ACL_DEFAULT.len()
  + XATTR_NAME_POSIX_ACL_DEFAULT.as_bytes().len()
  ),
  RafsV6XattrPrefix::new(
  XATTR_TRUSTED_PREFIX,
  EROFS_XATTR_INDEX_TRUSTED,
  - XATTR_TRUSTED_PREFIX.len()
  + XATTR_TRUSTED_PREFIX.as_bytes().len()
  ),
  RafsV6XattrPrefix::new(
  XATTR_SECURITY_PREFIX,
  EROFS_XATTR_INDEX_SECURITY,
  - XATTR_SECURITY_PREFIX.len()
  + XATTR_SECURITY_PREFIX.as_bytes().len()
  ),
  ];
  }
  