input: replace fallible integer cast with try_into

casting usize as i32 might wrap around in 32-bit targets, we don't
target them, but clippy complains.

Signed-off-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>

vhost-device-input/src/input.rs

@@ -64,7 +64,7 @@ macro_rules! ioctl_read_buf {
             for item in data.iter_mut() {
                 *item = $nr as u8;
             }
-            Ok(data.len() as i32)
+            Ok(data.len().try_into().unwrap())
         }
     )
 }