rust: fix single_binding

rust: fix single_binding

error: this match could be written as a `let` statement
   --> src/nfs/nfs3_records.rs:747:9
    |
747 | /         match result {
748 | |             (r, request) => {
749 | |                 assert_eq!(r.len(), 0);
750 | |                 assert_eq!(request.handle, expected_handle);
751 | |                 assert_eq!(request.name_vec, br#"bln"#);
752 | |             }
753 | |         }
    | |_________^