mac80211: don't inspect Sequence Control field on control frames

Per IEEE Std. 802.11-2012, Sec 8.2.4.4.1, the sequence Control field is
not present in control frames.  We noticed this problem when processing
Block Ack Requests.

Cc: stable@vger.kernel.org
Signed-off-by: Javier Cardona <javier@cozybit.com>
Signed-off-by: Javier Lopez <jlopex@cozybit.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/rx.c

@@ -1462,6 +1462,10 @@ ieee80211_rx_h_defragment(struct ieee80211_rx_data *rx)
 
 	hdr = (struct ieee80211_hdr *)rx->skb->data;
 	fc = hdr->frame_control;
+
+	if (ieee80211_is_ctl(fc))
+		return RX_CONTINUE;
+
 	sc = le16_to_cpu(hdr->seq_ctrl);
 	frag = sc & IEEE80211_SCTL_FRAG;