wifi: mac80211: prohibit deactivating all links

In the internal API this calls this is a WARN_ON, but that
should remain since internally we want to know about bugs
that may cause this. Prevent deactivating all links in the
debugfs write directly.

Reported-by: syzbot+0c5d8e65f23569a8ffec@syzkaller.appspotmail.com
Fixes: 3d90110 ("wifi: mac80211: implement link switching")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Link: https://patch.msgid.link/20241230091408.505bd125c35a.Ic3c1f9572b980a952a444cad62b09b9c6721732b@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/debugfs_netdev.c

@@ -727,7 +727,7 @@ static ssize_t ieee80211_if_parse_active_links(struct ieee80211_sub_if_data *sda
 {
 	u16 active_links;
 
-	if (kstrtou16(buf, 0, &active_links))
+	if (kstrtou16(buf, 0, &active_links) || !active_links)
 		return -EINVAL;
 
 	return ieee80211_set_active_links(&sdata->vif, active_links) ?: buflen;