rust: pin-init: examples: use Wrapper::pin_init instead of manual reimplementation

`UnsafeCell` gains the method via the extension trait `Wrapper`.

Link: https://patch.msgid.link/20260729-merge-init-v2-1-26adf47109e7@garyguo.net
Signed-off-by: Gary Guo <gary@garyguo.net>

rust/pin-init/examples/mutex.rs

@@ -79,11 +79,7 @@ pub fn new(val: impl PinInit<T>) -> impl PinInit<Self> {
             wait_list <- ListHead::new(),
             spin_lock: SpinLock::new(),
             locked: Cell::new(false),
-            data <- unsafe {
-                pin_init_from_closure(|slot: *mut UnsafeCell<T>| {
-                    val.__pinned_init(slot.cast::<T>())
-                })
-            },
+            data <- UnsafeCell::pin_init(val),
         })
     }