mac80211: use synchronize_rcu() with rcu_barrier()

mac80211: use synchronize_rcu() with rcu_barrier()

commit 8ceb59557bdc373e532b87d4142ce27e04218f0e upstream.

The RCU docs used to state that rcu_barrier() included a wait
for an RCU grace period; however the comments for rcu_barrier()
as of commit f0a0e6f... "rcu: Clarify memory-ordering properties
of grace-period primitives" contradict this.

So add back synchronize_{rcu,net}() to where they once were,
but keep the rcu_barrier()s for the call_rcu() callbacks.

Signed-off-by: Bob Copeland <bob@cozybit.com>
Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>