selftests: pmtu: Move route installs to a new function

Move the route add commands to a new function called setup_routing_old.
The '_old' refers to the classic way of installing routes.

Signed-off-by: David Ahern <dsahern@gmail.com>
Reviewed-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>