- 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.

Sponsored by:	Netflix
Sponsored by:	Nginx, Inc.
Remove a bunch of methods that are superseded by if_inc_counter().

Sponsored by:	Netflix
Sponsored by:	Nginx, Inc.
Fix the handling of EOP in status descriptors for if_igb(4) and don't
double-free mbufs.

Like ixgbe(4) chipsets, EOP is only set on the final descriptor
in a chain of descriptors.  So, to free the whole list of descriptors,
we should free the current slot _and_ the assembled list of descriptors
that make up the fragment list.

The existing code was setting discard once it saw EOP + an error status;
it then freed all the subsequent descriptors until the next EOP. That's
totally the wrong order.