selftests/net: convert stress_reuseport_listen.sh to run it in unique namespace

Here is the test result after conversion.

 # ./stress_reuseport_listen.sh
 listen 24000 socks took 0.47714

Acked-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>