I don't think the kernel can get away with the current approach. Object sizes and counts on 64-bit should be 64-bit unless there's a verifiable reason they can get away with 32-bit. Having it use leak memory isn't okay, just much less bad than vulnerabilities exploitable beyond just denial of service.
Every 32-bit reference count should probably have a short comment explaining why it can't overflow on 64-bit... if that can't be written or it's too complicated to demonstrate, it probably needs to be 64-bit. It's one of many pervasive forms of integer overflows in the kernel... :(
