veth: Showing peer of veth type dev in ip link (kernel side)

ip link has ability to show extra information of net work device if
kernel provides sunh information. With this patch veth driver can
provide its peer ifindex information to ip command via netlink
interface.

Signed-off-by: Masatake YAMATO <yamato@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>