Merge branch 'ioam6-mcast-events'

Justin Iurman says:

====================
ioam6: netlink multicast event

v5:
 - remove the "must be the destination" check before sending an ioam6
   event
v4:
 - rebase on top of net merge
v3:
 - patchset was mistakenly superseded due to same cover title used for
   iproute2-next equivalent patch -> resend (renamed)
v2:
 - fix warnings

Add generic netlink multicast event support to ioam6 as another solution
to share IOAM data with user space. The other one being via IPv6 raw
sockets combined with ancillary data (or packet socket, if the listener
does not need the processing of the IOAM Option-Type, since the hook is
before in that case). This patchset focuses on the IOAM Pre-allocated
Trace (the only Option-Type currently supported in the kernel), and so
on IOAM "trace" events. See an example of a consumer here [1].

  [1] https://github.com/Advanced-Observability/ioam-agent-python/blob/netlink_event/ioam-agent.py
====================

Signed-off-by: David S. Miller <davem@davemloft.net>