ip6_tunnel: add collect_md mode to IPv6 tunnels

Similar to gre, vxlan, geneve tunnels allow IPIP6 and IP6IP6 tunnels
to operate in 'collect metadata' mode.
Unlike ipv4 code here it's possible to reuse ip6_tnl_xmit() function
for both collect_md and traditional tunnels.
bpf_skb_[gs]et_tunnel_key() helpers and ovs (in the future) are the users.
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Thomas Graf <tgraf@suug.ch>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/ip6_tunnel.h

@@ -23,6 +23,7 @@ struct __ip6_tnl_parm {
 	__u8 proto;		/* tunnel protocol */
 	__u8 encap_limit;	/* encapsulation limit for tunnel */
 	__u8 hop_limit;		/* hop limit for tunnel */
+	bool collect_md;
 	__be32 flowinfo;	/* traffic class and flowlabel for tunnel */
 	__u32 flags;		/* tunnel flags */
 	struct in6_addr laddr;	/* local tunnel end-point address */