rust: arc: split unsafe block, add missing comment

The new SAFETY comment style is taken from existing comments in `deref`
and `drop.

Reviewed-by: Alice Ryhl <[email protected]>
Reviewed-by: Andreas Hindborg <[email protected]>
Signed-off-by: Tamir Duberstein <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Miguel Ojeda <[email protected]>

Author: tamird

rust/kernel/sync/arc.rs

@@ -389,10 +389,14 @@ impl<T: ?Sized> AsRef<T> for Arc<T> {
 
 impl<T: ?Sized> Clone for Arc<T> {
     fn clone(&self) -> Self {
+        // SAFETY: By the type invariant, there is necessarily a reference to the object, so it is
+        // safe to dereference it.
+        let refcount = unsafe { self.ptr.as_ref() }.refcount.get();
+
         // INVARIANT: C `refcount_inc` saturates the refcount, so it cannot overflow to zero.
         // SAFETY: By the type invariant, there is necessarily a reference to the object, so it is
         // safe to increment the refcount.
-        unsafe { bindings::refcount_inc(self.ptr.as_ref().refcount.get()) };
+        unsafe { bindings::refcount_inc(refcount) };
 
         // SAFETY: We just incremented the refcount. This increment is now owned by the new `Arc`.
         unsafe { Self::from_inner(self.ptr) }