only include s3.etag xattr from goofys when considering cache validity
only include s3.etag xattr from goofys when considering cache validity

that way if the xattr is updated without etag changing we won't
invalidate the cache, that includes lifecycle that bumps an object's
storage-class

also faster (with goofys) because s3.etag can be derived from LIST

kahing/catfs,kahing/catfs,kahing/catfs