df: fix warnings from default_trait_access lint

src/uu/df/src/filesystem.rs

@@ -317,12 +317,12 @@ mod tests {
 
         fn mount_info_with_dev_name(mount_dir: &str, dev_name: Option<&str>) -> MountInfo {
             MountInfo {
-                dev_id: Default::default(),
+                dev_id: String::default(),
                 dev_name: dev_name.map(String::from).unwrap_or_default(),
-                fs_type: Default::default(),
+                fs_type: String::default(),
                 mount_dir: String::from(mount_dir),
-                mount_option: Default::default(),
-                mount_root: Default::default(),
+                mount_option: String::default(),
+                mount_root: String::default(),
                 remote: Default::default(),
                 dummy: Default::default(),
             }