kernel: fix crash in flow offload code when cleaning up unregistered hooks

Signed-off-by: Felix Fietkau <[email protected]>

target/linux/generic/hack-4.14/650-netfilter-add-xt_OFFLOAD-target.patch

@@ -216,7 +216,7 @@ Signed-off-by: Felix Fietkau <[email protected]>
 +
 +restart:
 +	hlist_for_each_entry(hook, &hooks, list) {
-+		if (hook->used)
++		if (hook->used || !hook->registered)
 +			continue;
 +
 +		hlist_del(&hook->list);