Offloading switch device drivers may be able to collect statistics of the
traffic taking place in the HW datapath that pertains to a certain soft
netdevice, such as VLAN. Add the necessary infrastructure to allow exposing
these statistics to the offloaded netdevice in question. The API was shaped
by the following considerations:

- Collection of HW statistics is not free: there may be a finite number of
  counters, and the act of counting may have a performance impact. It is
  therefore necessary to allow toggling whether HW counting should be done
  for any particular SW netdevice.

- As the drivers are loaded and removed, a particular device may get
  offloaded and unoffloaded again. At the same time, the statistics values
  need to stay monotonic (modulo the eventual 64-bit wraparound),
  increasing only to reflect traffic measured in the device.

  To that end, the netdevice keeps around a lazily-allocated copy of struct
  rtnl_link_stats64. Device drivers then contribute to the values kept
  therein at various points. Even as the driver goes away, the struct stays
  around to maintain the statistics values.

- Different HW devices may be able to count different things. The
  motivation behind this patch in particular is exposure of HW counters on
  Nvidia Spectrum switches, where the only practical approach to counting
  traffic on offloaded soft netdevices currently is to use router interface
  counters, and count L3 traffic. Correspondingly that is the statistics
  suite added in this patch.

  Other devices may be able to measure different kinds of traffic, and for
  that reason, the APIs are built to allow uniform access to different
  statistics suites.

- Because soft netdevices and offloading drivers are only loosely bound, a
  netdevice uses a notifier chain to communicate with the drivers. Several
  new notifiers, NETDEV_OFFLOAD_XSTATS_*, have been added to carry messages
  to the offloading drivers.

- Devices can have various conditions for when a particular counter is
  available. As the device is configured and reconfigured, the device
  offload may become or cease being suitable for counter binding. A
  netdevice can use a notifier type NETDEV_OFFLOAD_XSTATS_REPORT_USED to
  ping offloading drivers and determine whether anyone currently implements
  a given statistics suite. This information can then be propagated to user
  space.

  When the driver decides to unoffload a netdevice, it can use a
  newly-added function, netdev_offload_xstats_report_delta(), to record
  outstanding collected statistics, before destroying the HW counter.

This patch adds a helper, call_netdevice_notifiers_info_robust(), for
dispatching a notifier with the possibility of unwind when one of the
consumers bails. Given the wish to eventually get rid of the global
notifier block altogether, this helper only invokes the per-netns notifier
block.
Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>