Implement a limit on on the number of IPv6 reassembly

queues per bucket.

There is a hashing algorithm which should distribute IPv6 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 IPv6 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.inet6.ip6.maxfragbucketsize).

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

newlib/libc/sys/rtems/include/netinet6/in6.h

@@ -643,7 +643,8 @@ struct ip6_mtuinfo {
 #define	IPV6CTL_INTRDQMAXLEN	52	/* max length of direct IPv6 netisr
 					 * queue */
 #define	IPV6CTL_MAXFRAGSPERPACKET	53 /* Max fragments per packet */
-#define	IPV6CTL_MAXID		54
+#define	IPV6CTL_MAXFRAGBUCKETSIZE	54 /* Max reassembly queues per bucket */
+#define	IPV6CTL_MAXID		55
 #endif /* __BSD_VISIBLE */
 
 /*