CVE-2026-90237: netfilter: nft_ct: move custom expectation support to helper

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

netfilter: nftct: move custom expectation support to helper

Originally, the ct expectation support called nfcthelperextadd() for confirmed conntracks, which is invalid, triggering a splat. This was fixed by commit 1710eb913bdc ("netfilter: nftct: skip expectations for confirmed conntrack") which restricted it to unconfirmed conntracks.

However, early insertion of expectations into the expectations list when the conntrack is unconfirmed leads to stale entries pointing to the wrong hlisthead through .pprev due to ct extension reallocation.

Commit 7c9664351980 ("netfilter: move nat hlisthead to nfconn") moved the nat hlisthead to nfconn for this reason:

1. ... 2. When reallocation of extension area occurs we need to fixup the bysource hash head via hlistreplacercu.

I'd rather not increase the size of the struct nfconn for this feature has very limited scope: only one expectation can be created at a time given expectclash() will make nfctexpectrelated() reports EBUSY. For this reason, relax nfctexpectrelated() not to drop packets in case expectation creation fails, therefore, expectation creation becomes best effort.

To address this issue, add an internal ct helper and attach it to the conntrack entry to streamline the custom ct expectation support with existing ct helpers.

Expose a new nfconntrackhelperrelease() function to release the internal helper that is allocated and attached to the conntrack entry to create the custom expectations. The nftct module removal always waits for rcu grace period, then the NULL helper callback is observed after this.

This patch also restricts the creation of expectations to different helpers other than this custom helper that is created for this type of expectations.