netfilter: connlimit: use rbtree for per-host conntrack obj storage
With current match design every invocation of the connlimit_match
function means we have to perform (number_of_conntracks % 256) lookups
in the conntrack table [ to perform GC/delete stale entries ].
This is also the reason why ____nf_conntrack_find() in perf top has
> 20% cpu time per core.
This patch changes the storage to rbtree which cuts down the number of
ct objects that need testing.
When looking up a new tuple, we only test the connections of the host
objects we visit while searching for the wanted host/network (or
the leaf we need to insert at).
The slot count is reduced to 32. Increasing slot count doesn't
speed up things much because of rbtree nature.