This commit is a bit of a re-arrange of deck chairs. It
gets both rack and bbr ready for the completion of the STATs
framework in FreeBSD. For now if you don't have both NF_stats and
stats on it disables them. As soon as the rest of the stats framework
lands we can remove that restriction and then just uses stats when
defined.
Sponsored by: Netflix Inc.
Differential Revision: https://reviews.freebsd.org/D22479