iwl4965: fix skb usage after free

Since

commit a120e912eb51e347f36c71b60a1d13af74d30e83
Author: Stanislaw Gruszka <sgruszka@redhat.com>
Date:   Fri Feb 19 15:47:33 2010 -0800

    iwlwifi: sanity check before counting number of tfds can be free

we use skb->data after calling ieee80211_tx_status_irqsafe(), which
could free skb instantly.

On current kernels I do not observe practical problems related with
bug, but on 2.6.35.y it cause random system hangs when stressing
wireless link, making bisection of other problems impossible.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/iwlegacy/iwl-4965-tx.c

@@ -1127,12 +1127,16 @@ int iwl4965_tx_queue_reclaim(struct iwl_priv *priv, int txq_id, int index)
 	     q->read_ptr = iwl_legacy_queue_inc_wrap(q->read_ptr, q->n_bd)) {
 
 		tx_info = &txq->txb[txq->q.read_ptr];
-		iwl4965_tx_status(priv, tx_info,
-				 txq_id >= IWL4965_FIRST_AMPDU_QUEUE);
+
+		if (WARN_ON_ONCE(tx_info->skb == NULL))
+			continue;
 
 		hdr = (struct ieee80211_hdr *)tx_info->skb->data;
-		if (hdr && ieee80211_is_data_qos(hdr->frame_control))
+		if (ieee80211_is_data_qos(hdr->frame_control))
 			nfreed++;
+
+		iwl4965_tx_status(priv, tx_info,
+				 txq_id >= IWL4965_FIRST_AMPDU_QUEUE);
 		tx_info->skb = NULL;
 
 		priv->cfg->ops->lib->txq_free_tfd(priv, txq);