Replace cosqos with numa_domain in mbuf pkthdr

The cosqos field was added nearly 6 years ago in r254804, and it is
still unused by any in-tree consumers. I have a patchset that I'm
working on which aligns many network resources by NUMA domain,
including inps, inpcb lb group, tcp pacing, lagg output link
selection, backing pages for sendfile, and more. It reduces
cross-domain traffic by roughly 50% for a real web workload.

This patchset relies on being able to store the numa domain in the
mbuf, and grabbing the unused cosqos field for this purpose is the
first step in starting to usptream it.

Reviewed by: kib, markj
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D19862