Kernel/Net: Make the LoopbackAdapter initializer to use ErrorOr pattern

This looks much more nice, and also matches our pattern for other types
of network adapters' initializers.

Kernel/Net/LoopbackAdapter.h

@@ -15,7 +15,7 @@ private:
     LoopbackAdapter(NonnullOwnPtr<KString>);
 
 public:
-    static LockRefPtr<LoopbackAdapter> try_create();
+    static ErrorOr<NonnullRefPtr<LoopbackAdapter>> try_create();
     virtual ~LoopbackAdapter() override;
 
     virtual ErrorOr<void> initialize(Badge<NetworkingManagement>) override { VERIFY_NOT_REACHED(); }