if_pflog: Limit the maximum unit via the new KPI

The cloner has the ability to limit the maximum unit. Employ it to do
that rather than roll our own.

No functional change intended.

Reviewed by: kp
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D45767

(cherry picked from commit 07d138afc7e5efee73368459dd047493713056cf)