netfilter: conntrack: refine gc worker heuristics, redux

This further refines the changes made to conntrack gc_worker in
commit e0df8cae ("netfilter: conntrack: refine gc worker heuristics").

The main idea of that change was to reduce the scan interval when evictions
take place.

However, on the reporters' setup, there are 1-2 million conntrack entries
in total and roughly 8k new (and closing) connections per second.

In this case we'll always evict at least one entry per gc cycle and scan
interval is always at 1 jiffy because of this test:

 } else if (expired_count) {
     gc_work->next_gc_run /= 2U;
     next_run = msecs_to_jiffies(1);

being true almost all the time.

Given we scan ~10k entries per run its clearly wrong to reduce interval
based on nonzero eviction count, it will only waste cpu cycles since a vast
majorities of conntracks are not timed out.

Thus only look at the ratio (scanned entries vs. evicted entries) to make
a decision on whether to reduce or not.

Because evictor is supposed to only kick in when system turns idle after
a busy period, pick a high ratio -- this makes it 50%.  We thus keep
the idea of increasing scan rate when its likely that table contains many
expired entries.

In order to not let timed-out entries hang around for too long
(important when using event logging, in which case we want to timely
destroy events), we now scan the full table within at most
GC_MAX_SCAN_JIFFIES (16 seconds) even in worst-case scenario where all
timed-out entries sit in same slot.

I tested this with a vm under synflood (with
sysctl net.netfilter.nf_conntrack_tcp_timeout_syn_recv=3).

While flood is ongoing, interval now stays at its max rate
(GC_MAX_SCAN_JIFFIES / GC_MAX_BUCKETS_DIV -> 125ms).

With feedback from Nicolas Dichtel.

Reported-by: Denys Fedoryshchenko <nuclearcat@nuclearcat.com>
Cc: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Fixes: b87a2f91 ("netfilter: conntrack: add gc worker to remove timed-out entries")
Signed-off-by: Florian Westphal <fw@strlen.de>
Tested-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Tested-by: Denys Fedoryshchenko <nuclearcat@nuclearcat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>