pf: Slightly relax pf_rule_addr validation

Ensure we don't reject no-route / urpf-failed addresses.

PR: 253479
Approved by: re (gjb)
Reported by: michal AT microwave.sk
Revied by: donner@
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D28650

(cherry picked from commit 5e42cb139fc17f165c9c93ac97069dc7770490e2)
(cherry picked from commit f9a66bb91ae1f3f175d0c16730c683841525bd1d)