rt2x00: Fix panic on frame padding for rt2800 usb devices

Backtrace:
  rt2800usb_write_tx_data
  rt2x00queue_write_tx_frame
  rt2x00mac_tx
  invoke_tx_handlers
  __ieee80211_tx
  ieee80211_tx
  virt_to_head_page
  ieee80211_xmit
  ieee80211_tx_skb
  ieee80211_scan_work
  schedule
  ieee80211_scan_work
  process_one_work
  ...

It tried to expand the skb past it's end using skb_put. So I replaced it
with a call to skb_padto, which takes the issue into account.
Signed-off-by: Ismael Luceno <ismael.luceno@gmail.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/rt2x00/rt2800usb.c

@@ -369,7 +369,10 @@ static void rt2800usb_write_tx_desc(struct queue_entry *entry,
 static void rt2800usb_write_tx_data(struct queue_entry *entry,
 					struct txentry_desc *txdesc)
 {
-	u8 padding_len;
+	unsigned int len;
+	int err;
+
+	rt2800_write_tx_data(entry, txdesc);
 
 	/*
 	 * pad(1~3 bytes) is added after each 802.11 payload.
@@ -378,9 +381,14 @@ static void rt2800usb_write_tx_data(struct queue_entry *entry,
 	 * | TXINFO | TXWI | 802.11 header | L2 pad | payload | pad | USB end pad |
 	 *                 |<------------- tx_pkt_len ------------->|
 	 */
-        rt2800_write_tx_data(entry, txdesc);
-        padding_len = roundup(entry->skb->len + 4, 4) - entry->skb->len;
-        memset(skb_put(entry->skb, padding_len), 0, padding_len);
+	len = roundup(entry->skb->len, 4) + 4;
+	err = skb_padto(entry->skb, len);
+	if (unlikely(err)) {
+		WARNING(entry->queue->rt2x00dev, "TX SKB padding error, out of memory\n");
+		return;
+	}
+
+	entry->skb->len = len;
 }
 
 /*