- Use if_inc_counter() to increment various counters.
- Do not ever set a counter to a value. For those counters
  that we don't increment, but return directly from hardware
  create cases in if_get_counter() method.

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git-svn-id: svn+ssh://svn.freebsd.org/base/head@271782 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f