Allow clippy::unnecessary_fallible_conversions

FFI types like `c_long` vary on 32/64-bit, and the check is only needed on
64-bit conversions. Fixing this lint makes the code less readable.

```
error: use of a fallible conversion when an infallible one could be used
   -->
packages/modules/Virtualization/authfs/fd_server/src/aidl.rs:380:36
    |
380 |         blockSize: st.block_size().try_into()?,
    |                                    ^^^^^^^^ help: use: `into`
    |
    = help: for further information visit
https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_fallible_conversions
    = note: `-D clippy::unnecessary-fallible-conversions` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::unnecessary_fallible_conversions)]`
```

Bug: http://b/321303117
Test: toolchain/android_rust/test_compiler.py --prebuilt-path dist/rust-dev.tar.xz --target aosp_cf_x86_64_phone --all-rust
Change-Id: I04c0ac34b8e084250803e8eeb17e0a109b10cbbc
diff --git a/authfs/fd_server/src/aidl.rs b/authfs/fd_server/src/aidl.rs
index ada3ffb..8edd899 100644
--- a/authfs/fd_server/src/aidl.rs
+++ b/authfs/fd_server/src/aidl.rs
@@ -375,6 +375,10 @@
     }
 }
 
+// FFI types like `c_long` vary on 32/64-bit, and the check is only needed on
+// 64-bit conversions. Fixing this lint makes the code less readable.
+#[allow(unknown_lints)]
+#[allow(clippy::unnecessary_fallible_conversions)]
 fn try_into_fs_stat(st: Statvfs) -> Result<FsStat, std::num::TryFromIntError> {
     Ok(FsStat {
         blockSize: st.block_size().try_into()?,