selftests: pmtu: Add support for routing via nexthop objects

Add routing setup using nexthop objects and repeat tests with
old and new routing.

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