mac80211: Fix panic on fragmentation with power saving

It was possible to hit a kernel panic on NULL pointer dereference in
dev_queue_xmit() when sending power save buffered frames to a STA that
woke up from sleep. This happened when the buffered frame was requeued
for transmission in ap_sta_ps_end(). In order to avoid the panic, copy
the skb->dev and skb->iif values from the first fragment to all other
fragments.
Signed-off-by: NJouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

net/mac80211/tx.c

@@ -752,6 +752,8 @@ ieee80211_tx_h_fragment(struct ieee80211_tx_data *tx)
 		skb_copy_queue_mapping(frag, first);
 
 		frag->do_not_encrypt = first->do_not_encrypt;
+		frag->dev = first->dev;
+		frag->iif = first->iif;
 
 		pos += copylen;
 		left -= copylen;