Kernel: Mark a bunch of NonnullRefPtrs also const to ensure immutability

These were easy to pick-up as these pointers are assigned during the
construction point and are never changed afterwards.

This small change to these pointers will ensure that our code will not
accidentally assign these pointers with a new object which is always a
kind of bug we will want to prevent.

Kernel/FileSystem/FileBackedFileSystem.h

@@ -38,6 +38,6 @@ private:
     virtual bool is_file_backed() const override { return true; }
 
     IntrusiveListNode<FileBackedFileSystem> m_file_backed_file_system_node;
-    NonnullRefPtr<OpenFileDescription> m_file_description;
+    NonnullRefPtr<OpenFileDescription> const m_file_description;
 };
 }