rust: pin-init: internal: generate brace in macro for init code blocks

`init!` support interleaving code execution and initialization, and code
execution is done using `_: { ... }` syntax. If the code inside block is a
single statement, Rust may add a lint about unused braces, but the
suggestion will be incorrect as block is required by pin-init.

Currently we use `unused_brace` to suppress this, but this affect
everything nested inside as well. Use an alternative approach by generating
the block from the macro, then rustc will know to not emit the lint.

Reviewed-by: Benno Lossin <lossin@kernel.org>
Link: https://patch.msgid.link/20260710-pin-init-sync-v1-5-8fa16cde87ae@garyguo.net
Signed-off-by: Gary Guo <gary@garyguo.net>

rust/pin-init/internal/src/init.rs

@@ -233,10 +233,12 @@ fn init_fields(
             InitializerKind::Value { ident, .. } => ident,
             InitializerKind::Init { ident, .. } => ident,
             InitializerKind::Code { block, .. } => {
+                let stmt = &block.stmts;
                 res.extend(quote! {
                     #(#attrs)*
-                    #[allow(unused_braces)]
-                    #block
+                    {
+                        #(#stmt)*
+                    }
                 });
                 continue;
             }