mac80211: fix two remote exploits

Lennert Buytenhek noticed a remotely triggerable problem
in mac80211, which is due to some code shuffling I did
that ended up changing the order in which things were
done -- this was in

  commit d75636ef9c
  Author: Johannes Berg <johannes@sipsolutions.net>
  Date:   Tue Feb 10 21:25:53 2009 +0100

    mac80211: RX aggregation: clean up stop session

The problem is that the BUG_ON moved before the various
checks, and as such can be triggered.

As the comment indicates, the BUG_ON can be removed since
the ampdu_action callback must already exist when the
state is OPERATIONAL.

A similar code path leads to a WARN_ON in
ieee80211_stop_tx_ba_session, which can also be removed.

Cc: stable@kernel.org [2.6.29+]
Cc: Lennert Buytenhek <buytenh@marvell.com>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/mac80211/agg-rx.c

@@ -85,10 +85,6 @@ void ieee80211_sta_stop_rx_ba_session(struct ieee80211_sub_if_data *sdata, u8 *r
 	struct ieee80211_local *local = sdata->local;
 	struct sta_info *sta;
 
-	/* stop HW Rx aggregation. ampdu_action existence
-	 * already verified in session init so we add the BUG_ON */
-	BUG_ON(!local->ops->ampdu_action);
-
 	rcu_read_lock();
 
 	sta = sta_info_get(local, ra);

net/mac80211/agg-tx.c

@@ -545,7 +545,7 @@ int ieee80211_stop_tx_ba_session(struct ieee80211_hw *hw,
 	struct sta_info *sta;
 	int ret = 0;
 
-	if (WARN_ON(!local->ops->ampdu_action))
+	if (!local->ops->ampdu_action)
 		return -EINVAL;
 
 	if (tid >= STA_TID_NUM)