message: rafs: use RafsMode::from_str to set mode
  RafsMode has its from_str method to convert a str, use this
  method to simply code, not use a match to convert it manually.
diff: @@ -453,17 +453,11 @@ impl Default for RafsSuper {
  impl RafsSuper {
  /// Create a new `RafsSuper` instance from a `RafsConfig` object.
  pub fn new(conf: &RafsConfig) -> Result<Self> {
  - let mut rs = Self::default();
  -
  - match conf.mode.as_str() {
  - "direct" => rs.mode = RafsMode::Direct,
  - "cached" => rs.mode = RafsMode::Cached,
  - _ => return Err(einval!("Rafs mode should be 'direct' or 'cached'")),
  - }
  -
  - rs.validate_digest = conf.digest_validate;
  -
  - Ok(rs)
  + Ok(Self {
  + mode: RafsMode::from_str(&conf.mode.as_str())?,
  + validate_digest: conf.digest_validate,
  + ..Default::default()
  + })
  }
  /// Destroy the filesystem super block.
  