selftest: txtimestamp: fix net ns entry logic

According to 'man 8 ip-netns', if `ip netns identify` returns an empty string,
there's no net namespace associated with current PID: fix the net ns entrance
logic.

Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

tools/testing/selftests/net/txtimestamp.sh

@@ -75,7 +75,7 @@ main() {
 	fi
 }
 
-if [[ "$(ip netns identify)" == "root" ]]; then
+if [[ -z "$(ip netns identify)" ]]; then
 	./in_netns.sh $0 $@
 else
 	main $@