pf: factor out rule counter update code

Break out rule counter update code into a separate function, makes the
behaviour consistent between IPv4 and IPv6.

From martin.pelikan@gmail.com

Obtained from: OpenBSD, mcbride <mcbride@openbsd.org>, ce38da5678
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D46590