pf: Limit the fragment entry queue length to 64 per bucket.

So we have a global limit of 1024 fragments, but it is fine grained to
the region of the packet. Smaller packets may have less fragments.
This costs another 16 bytes of memory per reassembly and devides the
worst case for searching by 8.

Obtained from: OpenBSD
Differential Revision: https://reviews.freebsd.org/D17734