pf: add COMPAT_FREEBSD13 for DIOCKEEPCOUNTERS

DIOCKEEPCOUNTERS used to overlap with DIOCGIFSPEEDV0, which has been
fixed in 14, but remains in stable/12 and stable/13.
Support the old, overlapping, call under COMPAT_FREEBSD13.

Reviewed by: jhb
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D33001