mac80211: add missing rcu_barrier

mac80211: add missing rcu_barrier

mac80211 uses call_rcu() with functions that are
defined in the module, so it must use rcu_barrier()
at module exit time.

Luckily, this seems to not be a problem in practice
as module unload and unregistration takes a long
time and probably does multiple synchronize_rcu().

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>