MFC r337780:

MFC r337780:
  Implement a limit on on the number of IPv4 reassembly queues per bucket.

  There is a hashing algorithm which should distribute IPv4 reassembly
  queues across the available buckets in a relatively even way. However,
  if there is a flaw in the hashing algorithm which allows a large number
  of IPv4 fragment reassembly queues to end up in a single bucket, a per-
  bucket limit could help mitigate the performance impact of this flaw.

  Implement such a limit, with a default of twice the maximum number of
  reassembly queues divided by the number of buckets. Recalculate the
  limit any time the maximum number of reassembly queues changes.
  However, allow the user to override the value using a sysctl
  (net.inet.ip.maxfragbucketsize).

Approved by:	so
Security:	FreeBSD-SA-18:10.ip
Security:	CVE-2018-6923