rust: macros: fix redefine const_name in vtable

If the trait has same function name, the `vtable` macro
will redefine its `gen_const_name`, e.g.:
```rust
    #[vtable]
    pub trait Foo {
        #[cfg(CONFIG_X)]
        fn bar();

        #[cfg(not(CONFIG_X))]
        fn bar(x: usize);
    }
```
Use `HashSet` to avoid this.

Signed-off-by: Qingsong Chen <changxian.cqs@antgroup.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Link: https://lore.kernel.org/r/20230626074242.3945398-2-changxian.cqs@antgroup.com

Author: cqs21

rust/macros/vtable.rs

@@ -27,7 +27,7 @@ pub(crate) fn vtable(_attr: TokenStream, ts: TokenStream) -> TokenStream {
     };
 
     let mut body_it = body.stream().into_iter();
-    let mut functions = Vec::new();
+    let mut functions = HashSet::new();
     let mut consts = HashSet::new();
     while let Some(token) = body_it.next() {
         match token {
@@ -37,7 +37,7 @@ pub(crate) fn vtable(_attr: TokenStream, ts: TokenStream) -> TokenStream {
                     // Possibly we've encountered a fn pointer type instead.
                     _ => continue,
                 };
-                functions.push(fn_name);
+                functions.insert(fn_name);
             }
             TokenTree::Ident(ident) if ident.to_string() == "const" => {
                 let const_name = match body_it.next() {