lagg: stop double-counting output errors and counting drops as errors

Before this change, lagg double-counted errors from lagg members, and counted
every drop by a lagg member as an error. Eg, if lagg sent a packet, and the
underlying hardware driver dropped it, a counter would be incremented by both
lagg and the underlying driver.

This change attempts to fix that by incrementing lagg's counters only for
errors that do not come from underlying drivers.

Reviewed by: hselasky, jhb
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D24331