pf: Change pf_krule counters to use counter_u64

This improves the cache behaviour of pf and results in improved
throughput.

MFC after: 2 weeks
Sponsored by: Orange Business Services
Differential Revision: https://reviews.freebsd.org/D27760

(cherry picked from commit c3adacdad4d72b045058cd932351b94579cdd543)