lwtunnel: rename ip lwtunnel attributes

We already have IFLA_IPTUN_ netlink attributes. The IP_TUN_ attributes look
very similar, yet they serve very different purpose. This is confusing for
anyone trying to implement a user space tool supporting lwt.

As the IP_TUN_ attributes are used only for the lightweight tunnels, prefix
them with LWTUNNEL_IP_ instead to make their purpose clear. Also, it's more
logical to have them in lwtunnel.h together with the encap enum.

Fixes: 3093fbe7 ("route: Per route IP tunnel metadata via lightweight tunnel")
Signed-off-by: Jiri Benc <jbenc@redhat.com>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>