pf: Use counter(9) for pf_state byte/packet tracking

This improves cache behaviour by not writing to the same variable from
multiple cores simultaneously.

pf_state is only used in the kernel, so can be safely modified.

Reviewed by: Lutz Donnerhacke, philip
MFC after: 1 week
Sponsed by: Orange Business Services
Differential Revision: https://reviews.freebsd.org/D27661