ath9k_htc: Restore skb headroom when returning skb to mac80211

ath9k_htc adds padding between the 802.11 header and the payload during
TX by moving the header. When handing the frame back to mac80211 for TX
status handling the header is not moved back into its original position.
This can result in a too small skb headroom when entering ath9k_htc
again (due to a soft retransmission for example) causing an
skb_under_panic oops.

Fix this by moving the 802.11 header back into its original position
before returning the frame to mac80211 as other drivers like rt2x00
or ath5k do.

Reported-by: Marc Kleine-Budde <mkl@blackshift.org>
Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Tested-by: Marc Kleine-Budde <mkl@blackshift.org>
Signed-off-by: Marc Kleine-Budde <mkl@blackshift.org>
Cc: stable@vger.kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/ath/ath9k/htc_drv_txrx.c

@@ -448,6 +448,7 @@ static void ath9k_htc_tx_process(struct ath9k_htc_priv *priv,
 	struct ieee80211_conf *cur_conf = &priv->hw->conf;
 	bool txok;
 	int slot;
+	int hdrlen, padsize;
 
 	slot = strip_drv_header(priv, skb);
 	if (slot < 0) {
@@ -504,6 +505,15 @@ send_mac80211:
 
 	ath9k_htc_tx_clear_slot(priv, slot);
 
+	/* Remove padding before handing frame back to mac80211 */
+	hdrlen = ieee80211_get_hdrlen_from_skb(skb);
+
+	padsize = hdrlen & 3;
+	if (padsize && skb->len > hdrlen + padsize) {
+		memmove(skb->data + padsize, skb->data, hdrlen);
+		skb_pull(skb, padsize);
+	}
+
 	/* Send status to mac80211 */
 	ieee80211_tx_status(priv->hw, skb);
 }