message: nydusd: use BackFileSystem in fuse_backend_rs crate
  Remove the self defined BackFileSystem and use the one
  in the fuse_backend_rs crate.
diff: @@ -10,7 +10,7 @@ use std::path::PathBuf;
  use std::str::FromStr;
  use std::sync::{Arc, MutexGuard};
  -use fuse_backend_rs::api::{BackendFileSystem, Vfs};
  +use fuse_backend_rs::api::{BackFileSystem, Vfs};
  #[cfg(target_os = "linux")]
  use fuse_backend_rs::passthrough::{Config, PassthroughFs};
  use nydus::{FsBackendDesc, FsBackendType};
  @@ -23,9 +23,6 @@ use crate::daemon::DaemonResult;
  use crate::upgrade::{self, UpgradeManager};
  use crate::DaemonError;
  -//TODO: Try to public below type from fuse-rs thus no need to redefine it here.
  -type BackFileSystem = Box<dyn BackendFileSystem<Inode = u64, Handle = u64> + Send + Sync>;
  -
  /// Command to mount a filesystem.
  #[derive(Clone)]
  pub struct FsBackendMountCmd {
  