netfilter: flowtable: fix offloaded ct timeout never being extended

netfilter: flowtable: fix offloaded ct timeout never being extended

OpenWrt has recently migrated many platforms to kernel 6.18. On the
MediaTek platform, which supports hardware network offloading, WiFi
connections accelerated via the WED path were observed to drop after
roughly 300 seconds.

After several debugging sessions, assisted by the Claude LLM, the
problem was narrowed down as follows:

nf_flow_table_extend_ct_timeout() extends ct->timeout for offloaded
flows using:

	cmpxchg(&ct->timeout, expires, new_timeout);

'expires' comes from nf_ct_expires(ct) and is a relative value, while
ct->timeout holds an absolute timestamp. The two are never equal, so
the cmpxchg always fails and the timeout is never extended.

This goes unnoticed for most flows, but a long-lived hardware (WED)
offloaded flow on MediaTek MT7986 eventually has ct->timeout decay to
zero, the conntrack entry is reaped and the connection breaks.

Open-code the relative value from a single READ_ONCE(ct->timeout)
snapshot and compare against that same absolute snapshot in the
cmpxchg, so the timeout extension actually takes effect while the
datapath remains authoritative if it updates ct->timeout concurrently.

Fixes: 03428ca5cee9 ("netfilter: conntrack: rework offload nf_conn timeout extension logic")
Cc: stable@vger.kernel.org
Suggested-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Adrian Bente <adibente@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>