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>