message: nydusd: gc unused blob files after umounting
  After umounting a filesystem, try to gc unused blob files, otherwise
  it may break the gc logic in nydus-snapshotter.
diff: @@ -17,6 +17,7 @@ use nydus::{FsBackendDesc, FsBackendType};
  use rafs::fs::{Rafs, RafsConfig};
  use rafs::{trim_backend_config, RafsError, RafsIoRead};
  use serde::{self, Deserialize, Serialize};
  +use storage::factory::BLOB_FACTORY;
  use crate::daemon::DaemonResult;
  use crate::upgrade::{self, UpgradeManager};
  @@ -149,6 +150,9 @@ pub trait FsService: Send + Sync {
  upgrade::remove_mounts_state(&mut mgr_guard, cmd)?;
  }
  + debug!("try to gc unused blobs");
  + BLOB_FACTORY.gc(None);
  +
  Ok(())
  }
  