CVE-2026-80668: netfilter: nf_conntrack_expect: use conntrack GC to reap expectations

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

netfilter: nfconntrackexpect: use conntrack GC to reap expectations

This patch replaces the timer API by GC worker approach for expectations, as it already happened in many other subsystems.

Use the existing conntrack GC worker to iterate over the local list of expectations in the master conntrack to reap expired expectations. Check IPSHELPERBIT to run GC for expectations, set it on for nftct expectation which nevers sets it. Hold the expectation spinlock while iterating over the master conntrack expectation list to synchronize with nfctremoveexpectations(). This also performs runtime packet path garbage collection through the expectation insertion and lookup functions while walking over one of the chains of the global expectation hashtables. Unconfirmed conntrack entries are skipped since ct->ext can be reallocated and dying are skipped since those will be gone soon. Set on IPSHELPERBIT if the helper ct extension is added, then the new GC worker does not need to bump the ct refcount to check if the ct->ext helper is available.

This removes the extra bump on the refcount for expectation timers, this allows to remove several nfctexpectput() calls after the unlink, after this update only refcount remains at 1 while on the expectation hashes.

This patch implicitly addresses a race with the existing timer API allowing an expectation to access a stale exp->master pointer which has been already released when expectation removal loses races with an expiring timer, ie. timerdel() reporting false.

Add a new NFCTEXPECTDEAD flag to reap this expectation via GC. This is needed by nfconntrackunexpectrelated() which is called in error paths to invalidate newly created expectations that has been added into the hashes. These expectactions cannot be inmediately released as GC or nfctremoveexpectations() could race to make it. On expectation insert, the runtime GC reaps stale expectations before checking the expectation limit set by policy.

Set current timestamp in nfctexpectalloc(), then add the expectation policy timeout (or custom timeout specified added on top of this) to specify the expectation lifetime.