pf: try to lookup the icmp state based on a correct packet descriptor

MFC after: 1 week
Obtained from: OpenBSD, mikeb <mikeb@openbsd.org>, e467ea25dcd3
Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit b8cd169efa6ac0899b4998898129765ae5c685a6)
(cherry picked from commit fa4b64836183c33631d92dadb073a9e435c5bf6d)

Approved-by: re (cperciva)