rust_out_of_tree: change example data to a `Vec`

This follows the contents of `samples/rust/rust_minimal.rs` in v9, which
is the trimmed down version of v8 where `String` is not in `alloc` anymore.

Now, the log looks like:

    [    1.076945] rust_out_of_tree: Rust out-of-tree sample (init)
    [    1.084944] rust_out_of_tree: My numbers are [72, 108, 200]
    [    1.085944] rust_out_of_tree: Rust out-of-tree sample (exit)

Link: https://lore.kernel.org/lkml/20220805154231.31257-27-ojeda@kernel.org/
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust_out_of_tree.rs

@@ -13,22 +13,25 @@ module! {
 }
 
 struct RustOutOfTree {
-    message: String,
+    numbers: Vec<i32>,
 }
 
 impl kernel::Module for RustOutOfTree {
     fn init(_name: &'static CStr, _module: &'static ThisModule) -> Result<Self> {
         pr_info!("Rust out-of-tree sample (init)\n");
 
-        Ok(RustOutOfTree {
-            message: "on the heap!".try_to_owned()?,
-        })
+        let mut numbers = Vec::new();
+        numbers.try_push(72)?;
+        numbers.try_push(108)?;
+        numbers.try_push(200)?;
+
+        Ok(RustOutOfTree { numbers })
     }
 }
 
 impl Drop for RustOutOfTree {
     fn drop(&mut self) {
-        pr_info!("My message is {}\n", self.message);
+        pr_info!("My numbers are {:?}\n", self.numbers);
         pr_info!("Rust out-of-tree sample (exit)\n");
     }
 }