Fix clippy::unnecessary_cast warning

```
warning: casting to the same type is unnecessary (`u64` -> `u64`)
   --> src/lib.rs:398:33
    |
398 |         Poll::Ready(res.map(|p| p as u64))
    |                                 ^^^^^^^^ help: try: `p`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_cast
    = note: `#[warn(clippy::unnecessary_cast)]` on by default

warning: casting to the same type is unnecessary (`u64` -> `u64`)
   --> src/lib.rs:421:33
    |
421 |         Poll::Ready(res.map(|p| p as u64))
    |                                 ^^^^^^^^ help: try: `p`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_cast
```

src/lib.rs

@@ -395,7 +395,7 @@ impl<T: tokio::io::AsyncSeek> futures_io::AsyncSeek for Compat<T> {
         }
         let res = ready!(self.as_mut().project().inner.poll_complete(cx));
         *self.as_mut().project().seek_pos = None;
-        Poll::Ready(res.map(|p| p as u64))
+        Poll::Ready(res)
     }
 }
 
@@ -418,7 +418,7 @@ impl<T: futures_io::AsyncSeek> tokio::io::AsyncSeek for Compat<T> {
         };
         let res = ready!(self.as_mut().project().inner.poll_seek(cx, pos));
         *self.as_mut().project().seek_pos = None;
-        Poll::Ready(res.map(|p| p as u64))
+        Poll::Ready(res)
     }
 }