message: rafs: provide open/opendir
  To ask kernel to cache file and directory contents instead of dropping
  them across open/close.
diff: @@ -664,6 +664,17 @@ impl FileSystem for Rafs {
  r
  }
  + fn open(
  + &self,
  + _ctx: &Context,
  + _inode: Self::Inode,
  + _flags: u32,
  + _fuse_flags: u32,
  + ) -> Result<(Option<Self::Handle>, OpenOptions)> {
  + // Keep cache since we are readonly
  + Ok((None, OpenOptions::KEEP_CACHE))
  + }
  +
  fn release(
  &self,
  _ctx: &Context,
  @@ -793,6 +804,16 @@ impl FileSystem for Rafs {
  })
  }
  + fn opendir(
  + &self,
  + _ctx: &Context,
  + _inode: Self::Inode,
  + _flags: u32,
  + ) -> Result<(Option<Self::Handle>, OpenOptions)> {
  + // Cache dir since we are readonly
  + Ok((None, OpenOptions::CACHE_DIR))
  + }
  +
  fn releasedir(&self, _ctx: &Context, _inode: u64, _flags: u32, _handle: u64) -> Result<()> {
  Ok(())
  }
  