Convert HashMap<Key, OwnPtr<T>> to HashMap<Key, NonnullOwnPtr<T>>.

In every case I found, we never wanted to support null entry values.
With NonnullOwnPtr, we can encode that at the type level. :^)

Kernel/SharedBuffer.h

@@ -49,4 +49,4 @@ public:
     unsigned m_total_refs { 0 };
 };
 
-Lockable<HashMap<int, OwnPtr<SharedBuffer>>>& shared_buffers();
+Lockable<HashMap<int, NonnullOwnPtr<SharedBuffer>>>& shared_buffers();