Comment 2 for bug 1716964

Related doc ont he parameter:
https://www.kernel.org/doc/Documentation/networking/ip-sysctl.txt

There is not more than 0,1,2.

I agree to the suggested solution in general.
The issue applies to current dev back to trusty.

I'm not sure if in Artful with netplan and potential impacts on these hooks this needs an extra change.