CVE-2024-40993 netfilter: ipset: Fix suspicious rcu_dereference_protected()

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

netfilter: ipset: Fix suspicious rcu_dereference_protected()

When destroying all sets, we are either in pernet exit phase or
are executing a “destroy all sets command” from userspace. The latter
was taken into account in ip_set_dereference() (nfnetlink mutex is held),
but the former was not. The patch adds the required check to
rcu_dereference_protected() in ip_set_dereference().