netns: change wrong check for valid namespace
netns: change wrong check for valid namespace

lebauce/skydive,lebauce/skydive,skydive-project/skydive,skydive-project/skydive,safchain/skydive,lebauce/skydive,lebauce/skydive,skydive-project/skydive,skydive-project/skydive,lebauce/skydive,skydive-project/skydive,lebauce/skydive,safchain/skydive,safchain/skydive,safchain/skydive,skydive-project/skydive,skydive-project/skydive,safchain/skydive,safchain/skydive,lebauce/skydive,lebauce/skydive,safchain/skydive,safchain/skydive