uapi: IPv6 IOAM headers definition

This patch provides the IPv6 IOAM option header [1] as well as the IOAM
Trace header [2]. An IOAM option must be 4n-aligned. Here is an overview of
a Hop-by-Hop with an IOAM Trace option:

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Next header  |  Hdr Ext Len  |    Padding    |    Padding    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Option Type  |  Opt Data Len |    Reserved   |   IOAM Type   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         Namespace-ID          | NodeLen | Flags | RemainingLen|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                IOAM-Trace-Type                |    Reserved   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<-+
|                                                               |  |
|                         node data [n]                         |  |
|                                                               |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  D
|                                                               |  a
|                         node data [n-1]                       |  t
|                                                               |  a
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~                             ...                               ~  S
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  p
|                                                               |  a
|                         node data [1]                         |  c
|                                                               |  e
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|                                                               |  |
|                         node data [0]                         |  |
|                                                               |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<-+

The IOAM option header starts at "Option Type" and ends after "IOAM
Type". The IOAM Trace header starts at "Namespace-ID" and ends after
"IOAM-Trace-Type/Reserved".

IOAM Type: either Pre-allocated Trace (=0), Incremental Trace (=1),
Proof-of-Transit (=2) or Edge-to-Edge (=3). Note that both the
Pre-allocated Trace and the Incremental Trace look the same. The two
others are not implemented.

Namespace-ID: IOAM namespace identifier, not to be confused with network
namespaces. It adds further context to IOAM options and associated data,
and allows devices which are IOAM capable to determine whether IOAM
options must be processed or ignored. It can also be used by an operator
to distinguish different operational domains or to identify different
sets of devices.

NodeLen: Length of data added by each node. It depends on the Trace
Type.

Flags: Only the Overflow (O) flag for now. The O flag is set by a
transit node when there are not enough octets left to record its data.

RemainingLen: Remaining free space to record data.

IOAM-Trace-Type: Bit field where each bit corresponds to a specific kind
of IOAM data. See [2] for a detailed list.

  [1] https://tools.ietf.org/html/draft-ietf-ippm-ioam-ipv6-options
  [2] https://tools.ietf.org/html/draft-ietf-ippm-ioam-data



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