Use count(9) api for the bpf(4) statistics.

Currently each bfp descriptor uses u64 variables to maintain its counters.
On interfaces with high packet rate this leads to unnecessary contention
and inaccurate reporting.

PR: kern/205320
Reported by: elofu17 at hotmail.com
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D14726