selftest/tcp-ao: Set routes in a proper VRF table id

In unsigned-md5 selftests ip_route_add() is not needed in
client_add_ip(): the route was pre-setup in __test_init() => link_init()
for subnet, rather than a specific ip-address.

Currently, __ip_route_add() mistakenly always sets VRF table
to RT_TABLE_MAIN - this seems to have sneaked in during unsigned-md5
tests debugging. That also explains, why ip_route_add_vrf() ignored
EEXIST, returned by fib6.

Yet, keep EEXIST ignoring in bench-lookups selftests as it's expected
that those selftests may add the same (duplicate) routes.

Reported-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: David S. Miller <davem@davemloft.net>