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>

net: selftests: Stress reuseport listen

This patch adds a test that has 300 VIPs listening on port 443.
Each VIP:443 will have 80 listening socks by using SO_REUSEPORT.
Thus, it will have 24000 listening socks.

Before removing the port only listening_hash, all socks will be in the
same port 443 bucket and inet_reuseport_add_sock() spends much time to
walk through the bucket. After removing the port only listening_hash
and move all usage to the port+addr lhash2, each bucket in the
ideal case has 80 sk which is much smaller than before.

Here is the test result from a qemu:
Before: listen 24000 socks took 210.210485362 (~210s)
After: listen 24000 socks took 0.207173 (~210ms)

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>