[rust] Avoid warning for unused fields

Upstream rustc has expanded the unused field lint to to not count
usages performed by derived Clone and Debug implementations.
rust-lang/rust#85200

This CL marks such fields with an #[allow(unused)] tag.

Bug: 84550
Change-Id: Ie5a84dd7c941b47419eb2a011a8daed22fc16695
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/581062
Fuchsia-Auto-Submit: Adrian Danis <adanis@google.com>
Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com>
Reviewed-by: Tyler Mandry <tmandry@google.com>

src/lib/storage/storage_device/src/block_device.rs

@@ -21,6 +21,7 @@ struct VmoBufferSource {
     vmoid: VmoId,
     // This needs to be 'static because the BufferSource trait requires 'static.
     slice: UnsafeCell<&'static mut [u8]>,
+    #[allow(unused)]
     vmo: zx::Vmo,
 }