selftests/net: optmem_max became per netns

/proc/sys/net/core/optmem_max is now per netns, change two tests
that were saving/changing/restoring its value on the parent netns.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>