Kernel: Break retain cycle between Inode and VMObject.

There's no need for an Inode to keep its corresponding VMObject alive.
Obviously there are huge benefits to keeping a filesystem cache,
but leaking everything is hardly the right strategy. :^)

Kernel/FileSystem.cpp

@@ -148,7 +148,7 @@ void FS::sync()
     }
 }
 
-void Inode::set_vmo(RetainPtr<VMObject>&& vmo)
+void Inode::set_vmo(VMObject& vmo)
 {
-    m_vmo = move(vmo);
+    m_vmo = vmo.make_weak_ptr();
 }