Move if_opackets++ to where if_obytes are

[Ryota Ozaki <ozaki.ryota%gmail.com@localhost>]

Hi,

I propose to move if_opackets from device drivers to where
if_obytes is incremented (not just one place), for example
in if_transmit.

The aims of the change are:
- Unify how it's incremented
  - Usages of the counter are different between device
    drivers
    - There are roughly two places to count it up;
      (1) when a packet is dequeued from if_snd (in
      XXX_start function) and (2) when a packet is
      successfully transmitted (in Tx interrupt)
  - Users normally expect the counter is incremented in the
    same manner between device drivers
  - It is hard to unify the behavior while having the counter
    in each device driver
- Synchronize with if_obytes
  - Users normally expect if_opackets and if_obytes are
    synchronized, but they aren't now
  - It is hard to do so while having the two counters in
    different places
- Simplify the code
  - We can reduce the number of places doing if_opackets++

Some concerns on the change:
- The meaning/behavior of the counter is changed
  - From "the number of packets sent on the interface" to
    "the number of packets passed to the interface to send"
  - But if_obytes is ever like that
- if_oerrors isn't moved (at least in this change)
  - It is hard to count errors happening in a device
    driver outside the device driver
  - We can count the number of dropped packets at the if_snd
    queue but that's perhaps not what we want to know
    by if_oerrors
  - That aside IMO errors happening in a device driver should
    be counted by driver own statistic counters and be
    reported via sysctl, evcnt or the like
    - So we can know what kind of errors happened, not just
      some sort of errors happened

This is a patch of the change:
  http://www.netbsd.org/~ozaki-r/move-if_opackets.diff

The patch includes only changes for sys/net*, wm(4) and
vioif(4). Changes for other drivers will be made once the
proposal is accepted.

Any comments or suggestions?

Thanks,
  ozaki-r