Kernel: Use static_ptr_cast to convert between Userspace<T*> types

Some calls of copy_to_user were converting Userspace<T*> to
Userspace<U*> via the implicit conversion to FlatPtr. Change them to use
the static_ptr_cast overload that is designed to express this conversion

Kernel/Net/LocalSocket.cpp

@@ -425,7 +425,7 @@ ErrorOr<void> LocalSocket::ioctl(OpenFileDescription& description, unsigned requ
     switch (request) {
     case FIONREAD: {
         int readable = receive_buffer_for(description)->immediately_readable();
-        return copy_to_user(Userspace<int*>(arg), &readable);
+        return copy_to_user(static_ptr_cast<int*>(arg), &readable);
     }
     }