mac80211: fix RX A-MPDU session reorder timer deletion

mac80211: fix RX A-MPDU session reorder timer deletion

There's an issue with the way the RX A-MPDU reorder timer is
deleted that can cause a kernel crash like this:

 * tid_rx is removed - call_rcu(ieee80211_free_tid_rx)
 * station is destroyed
 * reorder timer fires before ieee80211_free_tid_rx() runs,
   accessing the station, thus potentially crashing due to
   the use-after-free

The station deletion is protected by synchronize_net(), but
that isn't enough -- ieee80211_free_tid_rx() need not have
run when that returns (it deletes the timer.) We could use
rcu_barrier() instead of synchronize_net(), but that's much
more expensive.

Instead, to fix this, add a field tracking that the session
is being deleted. In this case, the only re-arming of the
timer happens with the reorder spinlock held, so make that
code not rearm it if the session is being deleted and also
delete the timer after setting that field. This ensures the
timer cannot fire after ___ieee80211_stop_rx_ba_session()
returns, which fixes the problem.

Cc: stable@vger.kernel.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>