netfilter: nf_conntrack: fix crash when unloading helpers

Commit ea781f19 (netfilter: nf_conntrack: use SLAB_DESTROY_BY_RCU and)
get rid of call_rcu() was missing one conversion to the hlist_nulls
functions, causing a crash when unloading conntrack helper modules.
Reported-and-tested-by: Mariusz Kozlowski <m.kozlowski@tuxland.pl>
Signed-off-by: Patrick McHardy <kaber@trash.net>

net/netfilter/nf_conntrack_helper.c

@@ -176,7 +176,7 @@ static void __nf_conntrack_helper_unregister(struct nf_conntrack_helper *me,
 	}
 
 	/* Get rid of expecteds, set helpers to NULL. */
-	hlist_for_each_entry(h, nn, &net->ct.unconfirmed, hnnode)
+	hlist_nulls_for_each_entry(h, nn, &net->ct.unconfirmed, hnnode)
 		unhelp(h, me);
 	for (i = 0; i < nf_conntrack_htable_size; i++) {
 		hlist_nulls_for_each_entry(h, nn, &net->ct.hash[i], hnnode)