pf: fix regression in pflog output

pf_match_rule() must remember current anchor before descents
towards leaf. it must restore anchor as it ascents towards root.
Bug pointed out and fix also tested by Matthias Pitzl from genua.

OK bluhm@

Obtained from: OpenBSD, sashan <sashan@openbsd.org>, 73e0536f0e
Sponsored by: Rubicon Communications, LLC ("Netgate")