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().