o Remove struct if_data from struct ifnet. Now it is merely API structure
  for route(4) socket and ifmib(4) sysctl.
o Move fields from if_data to ifnet, but keep all statistic counters
  separate, since they should disappear later.
o Provide function if_data_copy() to fill if_data, utilize it in routing
  socket and ifmib handler.
o Provide overridable ifnet(9) method to fetch counters. If no provided,
  if_get_counters_compat() would be used, that returns old counters.

Sponsored by:	Netflix
Sponsored by:	Nginx, Inc.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@270870 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f