fix(fileio): satisfy ABI layout lint

Use the type's bit width for the pointer-sized tail in the source-file
projection layout assertion.  This is numerically identical to eight
pointer-sized bytes on supported targets while keeping the assertion
compatible with the repository's warnings-as-errors lint policy.

Test Plan:
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo fmt --manifest-path rust/Cargo.toml --all -- --check
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo clippy --manifest-path rust/Cargo.toml --all-targets -- -D warnings

rust/src/fileio_asyncio.rs

@@ -361,7 +361,7 @@ const _: () = {
     );
     assert!(
         size_of::<FIO_rust_compress_src_projection_t>()
-            == 3 * size_of::<usize>() + 4 * size_of::<c_int>() + 8 * size_of::<usize>()
+            == 3 * size_of::<usize>() + 4 * size_of::<c_int>() + usize::BITS as usize
     );
 };