ip_tunnel: Make ovs_tunnel_info and ovs_key_ipv4_tunnel generic

Rename the tunnel metadata data structures currently internal to
OVS and make them generic for use by all IP tunnels.

Both structures are kernel internal and will stay that way. Their
members are exposed to user space through individual Netlink
attributes by OVS. It will therefore be possible to extend/modify
these structures without affecting user ABI.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>