Generic networking statistics for netlink users
  ======================================================================
  
  Statistic counters are grouped into structs:
  
  Struct               TLV type              Description
  ----------------------------------------------------------------------
  gnet_stats_basic     TCA_STATS_BASIC       Basic statistics
  gnet_stats_rate_est  TCA_STATS_RATE_EST    Rate estimator
  gnet_stats_queue     TCA_STATS_QUEUE       Queue statistics
  none                 TCA_STATS_APP         Application specific
  
  
  Collecting:
  -----------
  
  Declare the statistic structs you need:
  struct mystruct {
  	struct gnet_stats_basic	bstats;
  	struct gnet_stats_queue	qstats;
  	...
  };

  Update statistics, in dequeue() methods only, (while owning qdisc->running)

  mystruct->tstats.packet++;
  mystruct->qstats.backlog += skb->pkt_len;
  
  
  Export to userspace (Dump):
  ---------------------------
  
  my_dumping_routine(struct sk_buff *skb, ...)
  {
  	struct gnet_dump dump;

  	if (gnet_stats_start_copy(skb, TCA_STATS2, &mystruct->lock, &dump,
  				  TCA_PAD) < 0)

  		goto rtattr_failure;
  
  	if (gnet_stats_copy_basic(&dump, &mystruct->bstats) < 0 ||
  	    gnet_stats_copy_queue(&dump, &mystruct->qstats) < 0 ||
  		gnet_stats_copy_app(&dump, &xstats, sizeof(xstats)) < 0)
  		goto rtattr_failure;
  
  	if (gnet_stats_finish_copy(&dump) < 0)
  		goto rtattr_failure;
  	...
  }
  
  TCA_STATS/TCA_XSTATS backward compatibility:
  --------------------------------------------
  
  Prior users of struct tc_stats and xstats can maintain backward
  compatibility by calling the compat wrappers to keep providing the
  existing TLV types.
  
  my_dumping_routine(struct sk_buff *skb, ...)
  {
      if (gnet_stats_start_copy_compat(skb, TCA_STATS2, TCA_STATS,

  				     TCA_XSTATS, &mystruct->lock, &dump,
  				     TCA_PAD) < 0)

  		goto rtattr_failure;
  	...
  }
  
  A struct tc_stats will be filled out during gnet_stats_copy_* calls
  and appended to the skb. TCA_XSTATS is provided if gnet_stats_copy_app
  was called.
  
  
  Locking:
  --------
  
  Locks are taken before writing and released once all statistics have
  been written. Locks are always released in case of an error. You
  are responsible for making sure that the lock is initialized.
  
  
  Rate Estimator:
  --------------
  
  0) Prepare an estimator attribute. Most likely this would be in user
     space. The value of this TLV should contain a tc_estimator structure.

     As usual, such a TLV needs to be 32 bit aligned and therefore the
     length needs to be appropriately set, etc. The estimator interval

     and ewma log need to be converted to the appropriate values.
     tc_estimator.c::tc_setup_estimator() is advisable to be used as the
     conversion routine. It does a few clever things. It takes a time
     interval in microsecs, a time constant also in microsecs and a struct
     tc_estimator to  be populated. The returned tc_estimator can be
     transported to the kernel.  Transfer such a structure in a TLV of type
     TCA_RATE to your code in the kernel.
  
  In the kernel when setting up:
  1) make sure you have basic stats and rate stats setup first.
  2) make sure you have initialized stats lock that is used to setup such
     stats.
  3) Now initialize a new estimator:
  
     int ret = gen_new_estimator(my_basicstats,my_rate_est_stats,
         mystats_lock, attr_with_tcestimator_struct);
  
     if ret == 0
         success
     else
         failed

  From now on, every time you dump my_rate_est_stats it will contain
  up-to-date info.

  
  Once you are done, call gen_kill_estimator(my_basicstats,
  my_rate_est_stats) Make sure that my_basicstats and my_rate_est_stats
  are still valid (i.e still exist) at the time of making this call.
  
  
  Authors:
  --------
  Thomas Graf <tgraf@suug.ch>
  Jamal Hadi Salim <hadi@cyberus.ca>