Various fixes to the stats in igb(4), ixgbe(4), and ixl(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).
• Fix a typo that in ixl(4) that caused output queue drops to be reported as input queue drops and input queue drops to be unreported.

Differential Revision: https://reviews.freebsd.org/D2402
Reviewed by: jfv, rstone (6)
Sponsored by: Norse Corp, Inc.