pf: use correct address family in pfr_pool_get()

pfr_pool_get() in call to pfr_prepare_network()must use af
instead of hardcoded AF_INET

Issues reported and patch submitted by:

Renaud Allard <renaud () allard ! it>

OK sashan@

Obtained from: OpenBSD, sashan <sashan@openbsd.org>, 8e156a5ebe
Sponsored by: Rubicon Communications, LLC ("Netgate")