netmap: fix build issue in unit test

This is a direct commit to fix a build issue introduced by
b74063f03a834e9f22fb46f8e989a9df19823ff0 (cherry-picked MFC).
The build failure is caused by the use of a netmap option
that is available in CURRENT but not in stable/13.
Fix it by using an option code that is available also in
stable/13. The option value is not relevant for the test.