ipv6: ioam: Support for Queue depth data field

v3:
 - Report 'backlog' (bytes) instead of 'qlen' (number of packets)

v2:
 - Fix sparse warning (use rcu_dereference)

This patch adds support for the queue depth in IOAM trace data fields.

The draft [1] says the following:

   The "queue depth" field is a 4-octet unsigned integer field.  This
   field indicates the current length of the egress interface queue of
   the interface from where the packet is forwarded out.  The queue
   depth is expressed as the current amount of memory buffers used by
   the queue (a packet could consume one or more memory buffers,
   depending on its size).

An existing function (i.e., qdisc_qstats_qlen_backlog) is used to
retrieve the current queue length without reinventing the wheel.

Note: it was tested and qlen is increasing when an artificial delay is
added on the egress with tc.

  [1] https://datatracker.ietf.org/doc/html/draft-ietf-ippm-ioam-data#section-5.4.2.7



Signed-off-by: Justin Iurman <justin.iurman@uliege.be>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>