netfilter: conntrack: tcp: use UNACK timeout for non-closing RST packets

In the Linux kernel, the following vulnerability has been resolved:

netfilter: conntrack: tcp: use UNACK timeout for non-closing RST packets

Commit be0502a3f2e9 ("netfilter: conntrack: tcp: only close if RST
matches exact sequence") keeps an established conntrack entry in
ESTABLISHED when an in-window RST does not match the expected sequence
number exactly, so the endpoint can validate the RST with a challenge
ACK.

The timeout selection nevertheless uses the CLOSE timeout for every RST
packet. The bug is that timeout selection is based on the packet type,
not on the state transition result: even when RST validation keeps
new_state in ESTABLISHED, the timeout is still forced to
TCP_CONNTRACK_CLOSE.

Linux TCP independently rate limits challenge ACKs per socket. A second
non-exact RST can therefore arrive after the first challenge ACK has
restored the timeout but before the rate limit expires. The second RST
lowers the timeout to 10 seconds again while the endpoint suppresses the
second challenge ACK, allowing the conntrack entry to expire while both
TCP endpoints remain established.

Using the ESTABLISHED timeout for such RSTs would avoid this short
expiration window, but it could also retain stale entries for the
five-day default because conntrack cannot reliably match the endpoint's
exact TCP state.

Use the UNACK timeout for RST packets that leave the conntrack entry in
TCP_CONNTRACK_ESTABLISHED. Exact-match RSTs and accepted RST packet
trains still fall through to timeouts[new_state], which preserves the
CLOSE timeout when conntrack accepts the RST as closing the flow.

This avoids the aggressive 10-second expiration window for non-exact
RSTs while preserving the short timeout for RSTs that conntrack accepts
as closing the flow.