netfilter: nft_ct: move custom expectation support to helper

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

netfilter: nft_ct: move custom expectation support to helper

Originally, the ct expectation support called nf_ct_helper_ext_add() for
confirmed conntracks, which is invalid, triggering a splat. This was
fixed by commit 1710eb913bdc ("netfilter: nft_ct: 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 hlist_head through .pprev due to ct extension reallocation.

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

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

I'd rather not increase the size of the struct nf_conn for this feature
has very limited scope: only one expectation can be created at a time
given expect_clash() will make nf_ct_expect_related() reports EBUSY.
For this reason, relax nf_ct_expect_related() 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 nf_conntrack_helper_release() function to release the
internal helper that is allocated and attached to the conntrack entry to
create the custom expectations. The nft_ct 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.