SinglyLinkedList: Implement find in terms of AK::find

Problem:
- The implementation of `find` is coupled to the implementation of
  `SinglyLinkedList`.

Solution:
- Decouple the implementation of `find` from the class by using a
  generic `find` algorithm.

Kernel/Devices/Device.h

@@ -75,7 +75,7 @@ public:
         {
             ScopedSpinLock lock(m_requests_lock);
             was_empty = m_requests.is_empty();
-            m_requests.append(RefPtr<AsyncDeviceRequest>(request));
+            m_requests.append(request);
         }
         if (was_empty)
             request->do_start({});