mac80211: reject zero cookie in mgmt-tx/roc cancel

When cancelling, you can cancel "any" (first in list) mgmt-tx
or remain-on-channel operation by using the value 0 for the
cookie along with the *opposite* operation, i.e.
 * cancel the first mgmt-tx by cancelling roc with 0 cookie
 * cancel the first roc by cancelling mgmt-tx with 0 cookie

This isn't really that bad since userspace should only pass
cookies that we gave it, but could lead to hard-to-debug
issues so better prevent it and reject zero values since we
never hand those out.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/offchannel.c

@@ -697,6 +697,9 @@ static int ieee80211_cancel_roc(struct ieee80211_local *local,
 	struct ieee80211_roc_work *roc, *tmp, *found = NULL;
 	int ret;
 
+	if (!cookie)
+		return -ENOENT;
+
 	mutex_lock(&local->mtx);
 	list_for_each_entry_safe(roc, tmp, &local->roc_list, list) {
 		if (!mgmt_tx && roc->cookie != cookie)