Merge branch 'net-sched-move-back-qlen-to-per-CPU-accounting'

Paolo Abeni says:

====================
net: sched: move back qlen to per CPU accounting

The commit 46b1c18f ("net: sched: put back q.qlen into a single location")
introduced some measurable regression in the contended scenarios for
lock qdisc.

As Eric suggested we could replace q.qlen access with calls to qdisc_is_empty()
in the datapath and revert the above commit. The TC subsystem updates
qdisc->is_empty in a somewhat loose way: notably 'is_empty' is set only when
the qdisc dequeue() calls return a NULL ptr. That is, the invocation after
the last packet is dequeued.

The above is good enough for BYPASS implementation - the only downside is that
we end up avoiding the optimization for a very small time-frame - but will
break hard things when internal structures consistency for classful qdisc
relies on child qdisc_is_empty().

A more strict 'is_empty' update adds a relevant complexity to its life-cycle, so
this series takes a different approach: we allow lockless qdisc to switch from
per CPU accounting to global stats accounting when the NOLOCK bit is cleared.
Since most pieces of infrastructure are already in place, this requires very
little changes to the pfifo_fast qdisc, and any later NOLOCK qdisc can hook
there with little effort - no need to maintain two different implementations.

The first 2 patches removes direct qlen access from non core TC code, the 3rd
and 4th patches place and use the infrastructure to allow stats account
switching and the 5th patch is the actual revert.

 v1 -> v2:
  - fixed build issues
  - more descriptive commit message for patch 5/5
====================

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