mac80211: fix reordering of buffered broadcast packets

BugLink: https://bugs.launchpad.net/bugs/1811077

commit 9ec1190d upstream.

If the buffered broadcast queue contains packets, letting new packets bypass
that queue can lead to heavy reordering, since the driver is probably throttling
transmission of buffered multicast packets after beacons.

Keep buffering packets until the buffer has been cleared (and no client
is in powersave mode).

Cc: stable@vger.kernel.org
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

net/mac80211/tx.c

@@ -431,8 +431,8 @@ ieee80211_tx_h_multicast_ps_buf(struct ieee80211_tx_data *tx)
 	if (ieee80211_hw_check(&tx->local->hw, QUEUE_CONTROL))
 		info->hw_queue = tx->sdata->vif.cab_queue;
 
-	/* no stations in PS mode */
-	if (!atomic_read(&ps->num_sta_ps))
+	/* no stations in PS mode and no buffered packets */
+	if (!atomic_read(&ps->num_sta_ps) && skb_queue_empty(&ps->bc_buf))
 		return TX_CONTINUE;
 
 	info->flags |= IEEE80211_TX_CTL_SEND_AFTER_DTIM;