rust: macros: fix usage of #[allow] in quote!

When using `quote!` as part of an expression that was not the last one
in a function, the `#[allow(clippy::vec_init_then_push)]` attribute
would be present on an expression, which is not allowed.
This patch refactors that part of the macro to use a statement instead.

Signed-off-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://lore.kernel.org/r/20230424081112.99890-1-benno.lossin@proton.me
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: BennoLossin

rust/macros/quote.rs

@@ -39,12 +39,14 @@ impl ToTokens for TokenStream {
 /// [`quote_spanned!`](https://docs.rs/quote/latest/quote/macro.quote_spanned.html) macro from the
 /// `quote` crate but provides only just enough functionality needed by the current `macros` crate.
 macro_rules! quote_spanned {
-    ($span:expr => $($tt:tt)*) => {
-    #[allow(clippy::vec_init_then_push)]
-    {
-        let mut tokens = ::std::vec::Vec::new();
-        let span = $span;
-        quote_spanned!(@proc tokens span $($tt)*);
+    ($span:expr => $($tt:tt)*) => {{
+        let mut tokens;
+        #[allow(clippy::vec_init_then_push)]
+        {
+            tokens = ::std::vec::Vec::new();
+            let span = $span;
+            quote_spanned!(@proc tokens span $($tt)*);
+        }
         ::proc_macro::TokenStream::from_iter(tokens)
     }};
     (@proc $v:ident $span:ident) => {};