selftests/net: fix in_netns.sh script

execute the subprocess in netns using 'ip netns exec'

Fixes: cc30c93f ("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>