mac80211: track only QoS data frames for admission control

For admission control, obviously all of that only works for
QoS data frames, otherwise we cannot even access the QoS
field in the header.

Syzbot reported (see below) an uninitialized value here due
to a status of a non-QoS nullfunc packet, which isn't even
long enough to contain the QoS header.

Fix this to only do anything for QoS data packets.

Reported-by: syzbot+614e82b88a1a4973e534@syzkaller.appspotmail.com
Fixes: 02219b3a ("mac80211: add WMM admission control support")
Link: https://lore.kernel.org/r/20211122124737.dad29e65902a.Ieb04587afacb27c14e0de93ec1bfbefb238cc2a0@changeidSigned-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/mlme.c

@@ -2452,11 +2452,18 @@ static void ieee80211_sta_tx_wmm_ac_notify(struct ieee80211_sub_if_data *sdata,
 					   u16 tx_time)
 {
 	struct ieee80211_if_managed *ifmgd = &sdata->u.mgd;
-	u16 tid = ieee80211_get_tid(hdr);
-	int ac = ieee80211_ac_from_tid(tid);
-	struct ieee80211_sta_tx_tspec *tx_tspec = &ifmgd->tx_tspec[ac];
+	u16 tid;
+	int ac;
+	struct ieee80211_sta_tx_tspec *tx_tspec;
 	unsigned long now = jiffies;
 
+	if (!ieee80211_is_data_qos(hdr->frame_control))
+		return;
+
+	tid = ieee80211_get_tid(hdr);
+	ac = ieee80211_ac_from_tid(tid);
+	tx_tspec = &ifmgd->tx_tspec[ac];
+
 	if (likely(!tx_tspec->admitted_time))
 		return;