FreeBSD: VID-d35373ae-4d34-11ee-8e38-002590c1f29c (CVE-2023-4809): FreeBSD -- pf incorrectly handles multiple IPv6 fragment headers

Problem Description: With a 'scrub fragment reassemble' rule, a packet containing multiple IPv6 fragment headers would be reassembled, and then immediately processed. That is, a packet with multiple fragment extension headers would not be recognized as the correct ultimate payload. Instead a packet with multiple IPv6 fragment headers would unexpectedly be interpreted as a fragmented packet, rather than as whatever the real payload is. Impact: IPv6 fragments may bypass firewall rules written on the assumption all fragments have been reassembled and, as a result, be forwarded or processed by the host.