selftests/net: add TFO key rotation selftest

Demonstrate how the primary and backup TFO keys can be rotated while
minimizing the number of client cookies that are rejected.

Signed-off-by: Jason Baron <jbaron@akamai.com>
Signed-off-by: Christoph Paasch <cpaasch@apple.com>
Acked-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>