mac80211: add missing synchronize_rcu

commit ad0e2b5a
Author: Johannes Berg <johannes.berg@intel.com>
Date:   Tue Jun 1 10:19:19 2010 +0200

    mac80211: simplify key locking

removed the synchronization against RCU and thus
opened a race window where we can use a key for
TX while it is already freed. Put a synchronisation
into the right place to close that window.

Reported-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Cc: stable@kernel.org [2.6.36+]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>