selftest/tcp-ao: Work on namespace-ified sysctl_optmem_max

Since commit f5769fae ("net: Namespace-ify sysctl_optmem_max")
optmem_max is per-netns, so need of switching to root namespace.
It seems trivial to keep the old logic working, so going to keep it for
a while (at least, until kernel with netns-optmem_max will be release).

Currently, there is a test that checks that optmem_max limit applies to
TCP-AO keys and a little benchmark that measures linked-list TCP-AO keys
scaling, those are fixed by this.

Cc: Eric Dumazet <edumazet@google.com>
Signed-off-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: David S. Miller <davem@davemloft.net>