siftr: bring back the siftr_pkts_per_log feature

Summary: this missing feature is introduced by commit aa61cff4249c

Test Plan: verified in Emulab.net

Reviewers: rscheff, tuexen
Approved by: tuexen (mentor)
Subscribers: imp, melifaro, glebius
Differential Revision: https://reviews.freebsd.org/D40336