uapi: ioam6: API for netlink multicast events

Add new api to support ioam6 events for generic netlink multicast. A
first "trace" event is added to the list of ioam6 events, which will
represent an IOAM Pre-allocated Trace Option-Type. It provides another
solution to share IOAM data with user space.

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

ipv6: ioam: IOAM Generic Netlink API

Add Generic Netlink commands to allow userspace to configure IOAM
namespaces and schemas. 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 ioam
Usage: ip ioam { COMMAND | help }
ip ioam namespace show
ip ioam namespace add ID [ data DATA32 ] [ wide DATA64 ]
ip ioam namespace del ID
ip ioam schema show
ip ioam schema add ID DATA
ip ioam schema del ID
ip ioam namespace set ID schema { ID | none }

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