downgrade network-manager

You can download them (booting on a live distrib, from another computer + usb stick, ...):
For amd64
http://archive.ubuntu.com/ubuntu/pool/main/libn/libnl3/libnl-3-200_3.2.21-1_amd64.deb
http://archive.ubuntu.com/ubuntu/pool/main/libn/libnl3/libnl-genl-3-200_3.2.21-1_amd64.deb
http://archive.ubuntu.com/ubuntu/pool/main/libn/libnl3/libnl-route-3-200_3.2.21-1_amd64.deb

For i386
http://archive.ubuntu.com/ubuntu/pool/main/libn/libnl3/libnl-3-200_3.2.21-1_i386.deb
http://archive.ubuntu.com/ubuntu/pool/main/libn/libnl3/libnl-genl-3-200_3.2.21-1_i386.deb
http://archive.ubuntu.com/ubuntu/pool/main/libn/libnl3/libnl-route-3-200_3.2.21-1_i386.deb

Then installing them with dpkg where you saved these .deb packages:
sudo dpkg -i libnl-*.deb

(re)starting NetworkManager should no more crash and brings back networking:
sudo service network-manager restart

As there isn't yet any upgrade on network-manager,
until this upgrade trigger in your package manager,
As mentioned in https://bugs.launchpad.net/ubuntu/+source/libnl3/+bug/1511735/comments/29

you may need to apply a hold until new NetworkManager is released:
sudo apt-mark hold libnl-3-200 libnl-genl-3-200 libnl-route-3-200

then when the update on NetworkManager triggers:
sudo apt-mark auto libnl-3-200 libnl-genl-3-200 libnl-route-3-200