Various fixes to the stats in igb(4) and ixgbe(4).
- Use hardware counters for ifnet stats in igb(4) when possible. This ensures these stats include packets that bypass the regular stack via netmap.
- Don't derefence values off the end of the igb(4) VF stats structure. Instead, add a dedicated if_get_counter method for igb(4) VF interfaces.
- Report missed packets on igb(4) as input queue drops rather than an input error.
- Report bug_ring drop counts as output queue drops for igb(4) and ixgbe(4).
- Export the buf_ring drop stats for individual rings via sysctl on ixgbe(4).