Working on change for: genetlink: make policy common to family

[Bruno Wolff III]

Wireguard isn't building on 5.2 right now because of commit:
3b0f31f2b8c9fb348e4530b88f6b64f9621f83d6 genetlink: make policy common to family

I've got Wireguard building, but need to do basic testing, then add a kernel 
version test in and do some other testing. If that all goes OK I'll submit 
a signed off patch to the list.

It looks to be a very simple change to netlink.c, but I could have easily 
missed something subtle.