oce(4): Account and trace mbufs before handing to hw

Once tx mbufs have been handed to hardware, nothing serializes the tx
path against completion and potential use-after-free of the outbound
mbuf. Perform accounting and BPF tap before queueing to hardware to
avoid this race.

Submitted by: Steve Wirtz <steve_wirtz AT dell.com>
Reviewed by: markj, rstone
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D25364