wifi: mac80211: exclude multicast packets from AQL pending airtime

In AP mode, multicast traffic is handled very differently from normal traffic,
especially if at least one client is in powersave mode.
This means that multicast packets can be buffered a lot longer than normal
unicast packets, and can eat up the AQL budget very quickly because of the low
data rate.
Along with the recent change to maintain a global PHY AQL limit, this can lead
to significant latency spikes for unicast traffic.

Since queueing multicast to hardware is currently not constrained by AQL limits
anyway, let's just exclude it from the AQL pending airtime calculation entirely.

Fixes: 8e4bac06 ("wifi: mac80211: add a per-PHY AQL limit to improve fairness")
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Link: https://lore.kernel.org/r/20220713083444.86129-1-nbd@nbd.nameSigned-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/tx.c

@@ -3760,7 +3760,7 @@ struct sk_buff *ieee80211_tx_dequeue(struct ieee80211_hw *hw,
 encap_out:
 	IEEE80211_SKB_CB(skb)->control.vif = vif;
 
-	if (vif &&
+	if (tx.sta &&
 	    wiphy_ext_feature_isset(local->hw.wiphy, NL80211_EXT_FEATURE_AQL)) {
 		bool ampdu = txq->ac != IEEE80211_AC_VO;
 		u32 airtime;