Raspberry pi fondation maintain a special kernel for raspberry pi. They patch it with a lot of stuff and security patch. Dirty cow is fixed even in 4.4.x version because they back port security patches.
Upstream kernel don’t support completely raspberry pi. Some developers are adding stuff to mainline kernel to have native support. Until then we must use raspberry pi fondation kernel.
Dirty cow is a joke, because there is a lot of security leaks in kernel, but because a lot of media speaks about it a lot of people are scare but there is hundred fix for security fix each year.