CVE-2026-72256: netfilter: xt_cluster: reject template conntracks in hash match

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

netfilter: xtcluster: reject template conntracks in hash match

xtclustermt() treats any non-NULL nfctget() result as a fully initialized conntrack and passes it to xtclusterhash().

This causes a state confusion bug when the raw table CT target attaches a template conntrack to skb->nfct before normal conntrack processing. Templates carry IPSTEMPLATE status but do not have a valid tuple for hashing yet, so xtclusterhash() can hit its WARNON() path on the zeroed l3num field.

Reject template conntracks before hashing them. This matches existing netfilter handling for template objects and avoids hashing incomplete conntrack state.