Comment 16 for bug 1709032

The panic appears to be fixed upstream via:

commit 9c3f3794926a997b1cab6c42480ff300efa2d162
Author: Liping Zhang <email address hidden>
Date: Sat Mar 25 16:35:29 2017 +0800

    netfilter: nf_ct_ext: fix possible panic after nf_ct_extend_unregister

    If one cpu is doing nf_ct_extend_unregister while another cpu is doing
    __nf_ct_ext_add_length, then we may hit BUG_ON(t == NULL). Moreover,
    there's no synchronize_rcu invocation after set nf_ct_ext_types[id] to
    NULL, so it's possible that we may access invalid pointer.
[...]