selftests/net: fix in_netns.sh script

execute the subprocess in netns using 'ip netns exec'

Fixes: cc30c93fa020 ("selftests/net: ignore background traffic in psock_fanout")
Signed-off-by: Prashant Bhole <bhole_prashant_q7@lab.ntt.co.jp>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

tools/testing/selftests/net/in_netns.sh

@@ -19,5 +19,5 @@ cleanup() {
 trap cleanup EXIT
 setup
 
-"$@"
+ip netns exec "${NETNS}" "$@"
 exit "$?"