samples/rust: add static mutex and condvar to sync sample

The sample includes local instances, this patch adds an example of how
to use static mutexes and conditional varibles.

Signed-off-by: Wedson Almeida Filho <wedsonaf@google.com>

Author: wedsonaf

samples/rust/rust_sync.rs

@@ -16,6 +16,11 @@ module! {
     license: b"GPL v2",
 }
 
+kernel::init_static_sync! {
+    static SAMPLE_MUTEX: Mutex<u32> = 10;
+    static SAMPLE_CONDVAR: CondVar;
+}
+
 struct RustSync;
 
 impl KernelModule for RustSync {
@@ -45,6 +50,16 @@ impl KernelModule for RustSync {
             cv.free_waiters();
         }
 
+        // Test static mutex + condvar.
+        *SAMPLE_MUTEX.lock() = 20;
+
+        {
+            let mut guard = SAMPLE_MUTEX.lock();
+            while *guard != 20 {
+                let _ = SAMPLE_CONDVAR.wait(&mut guard);
+            }
+        }
+
         // Test spinlocks.
         {
             // SAFETY: `init` is called below.