pkg/ip unit test: be agnostic of Linux version
pkg/ip unit test: be agnostic of Linux version

on Linux 4.4 the syscall error message is "invalid argument" not "file
exists"

Co-authored-by: Gabe Rosenhouse <657db4ad0dfa95e4e10afe2fdfba83b796a49c56@pivotal.io>

containernetworking/plugins,containernetworking/plugins