netfilter: nf_conncount: don't skip eviction when age is negative

commit 4cd273bb91b3001f623f516ec726c49754571b1a upstream.

(not in Linus's tree now, but in nf.git + linux-next.git already.)

age is signed integer, so result can be negative when the timestamps
have a large delta.  In this case we want to discard the entry.

Instead of using age >= 2 || age < 0, just make it unsigned.

Fixes: b36e4523 ("netfilter: nf_conncount: fix garbage collection confirm race")
Reviewed-by: Shawn Bohrer <sbohrer@cloudflare.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

[mfo: backport: use older file name, nf_conncount.c -> xt_connlimit.c]
Signed-off-by: Mauricio Faria de Oliveira <mfo@canonical.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>