ipv6: ioam: Support for IOAM injection with lwtunnels

Add support for the IOAM inline insertion (only for the host-to-host use case)
which is per-route configured with lightweight tunnels. The target is iproute2
and the patch is ready. It will be posted as soon as this patchset is merged.
Here is an overview:

$ ip -6 ro ad fc00::1/128 encap ioam6 trace type 0x800000 ns 1 size 12 dev eth0

This example configures an IOAM Pre-allocated Trace option attached to the
fc00::1/128 prefix. The IOAM namespace (ns) is 1, the size of the pre-allocated
trace data block is 12 octets (size) and only the first IOAM data (bit 0:
hop_limit + node id) is included in the trace (type) represented as a bitfield.

The reason why the in-transit (IPv6-in-IPv6 encapsulation) use case is not
implemented is explained on the patchset cover.

Signed-off-by: Justin Iurman <justin.iurman@uliege.be>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/ioam6_iptunnel.h

@@ -0,0 +1,13 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ *  IPv6 IOAM Lightweight Tunnel API
+ *
+ *  Author:
+ *  Justin Iurman <justin.iurman@uliege.be>
+ */
+#ifndef _LINUX_IOAM6_IPTUNNEL_H
+#define _LINUX_IOAM6_IPTUNNEL_H
+
+#include <uapi/linux/ioam6_iptunnel.h>
+
+#endif /* _LINUX_IOAM6_IPTUNNEL_H */