netlink: workaround veth being peered together through IFLA_LINK

netlink: workaround veth being peered together through IFLA_LINK

This is a change introduced in Linux 4.1. It's easy to detect, so,
workaround it. See:

    commit a45253bf32bf49cdb2807bad212b84f5ab51ac26
    Author: Nicolas Dichtel <nicolas.dichtel@6wind.com>
    Date:   Thu Apr 2 17:07:11 2015 +0200

        veth: set iflink to the peer veth

        Now that the peer netns is advertised in rtnl messages, we can set this property
        so that IFLA_LINK will advertise the peer ifindex. It allows the userland to get
        the full veth configuration.

        Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
        Signed-off-by: David S. Miller <davem@davemloft.net>