Lower the default limits on the IPv6 reassembly queue.

Currently, the limits are quite high. On machines with millions of
mbuf clusters, the reassembly queue limits can also run into
the millions. Lower these values.

Also, try to ensure that no bucket will have a reassembly
queue larger than approximately 100 items. This limits the cost to
find the correct reassembly queue when processing an incoming
fragment.

Due to the low limits on each bucket's length, increase the size of
the hash table from 64 to 1024.

Reviewed by: jhb
Security: FreeBSD-SA-18:10.ip
Security: CVE-2018-6923