Similar to the network stack issue fixed in r337782 pf did not limit the number
of fragments per packet, which could be exploited to generate high CPU loads
with a crafted series of packets.
Limit each packet to no more than 64 fragments. This should be sufficient on
typical networks to allow maximum-sized IP frames.
This addresses the issue for both IPv4 and IPv6.
MFC after: 3 days
Security: CVE-2018-5391
Sponsored by: Klara Systems
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