pf: Split the fragment reassembly queue into smaller parts

Remember 16 entry points based on the fragment offset. Instead of
a worst case of 8196 list traversals we now check a maximum of 512
list entries or 16 array elements.

Obtained from: OpenBSD
Differential Revision: https://reviews.freebsd.org/D17733

(cherry picked from commit fd2ea405e601bd5e240153c5de0f7c264946ce6f)