Fix empty root directories in mount namespace

This fixes an issue where previous sandbox's root directories created
for bind mounts would be available in new sandboxes as empty
directories.

While this doesn't cause any security issues, it causes tests to fail
and would likely be unexpected by consumers.

src/linux/namespaces.rs

@@ -64,10 +64,9 @@ fn create_mount_namespace(bind_mounts: HashMap<PathBuf, libc::c_ulong>) -> Resul
     let new_root = PathBuf::from(NEW_ROOT);
     let put_old = new_root.join(OLD_ROOT_DIR);
 
-    // Ensure new root exists.
-    if !new_root.exists() {
-        fs::create_dir(&new_root)?;
-    }
+    // Ensure new root is available as an empty directory.
+    fs::remove_dir_all(&new_root)?;
+    fs::create_dir(&new_root)?;
 
     // Create C-friendly versions for our paths.
     let new_root_c = CString::new(new_root.as_os_str().as_bytes()).unwrap();