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 <[e-mail]>
Acked-by: Willem de Bruijn <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

tools/testing/selftests/net/txtimestamp.sh

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