Task hang via unserialized conntrack cleanup

Denial of service in affected Linux kernels allows local users to hang tasks through concurrent conntrack resize and namespace cleanup. nf_ct_iterate_cleanup restarts its bucket scan whenever a concurrent conntrack hash resize occurs, so repeated resizes can indefinitely delay network namespace teardown. Exploitation requires local low-privileged access to create network namespaces and change conntrack hash sizes; the demonstrated result is an availability-only task hang.