ipsec: Fix a lock leak in ipsec_chkreplay()

Reported by: Chris Jarrett-Davies of the OpenAI Codex Security Team
Reviewed by: pouria, kp
Fixes: 0361f165f219 ("ipsec: replace SECASVAR mtx by rmlock")
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D58521