percpu-rwsem: Replace synchronize_sched() with synchronize_rcu()

Now that synchronize_rcu() waits for preempt-disable regions of code
as well as RCU read-side critical sections, synchronize_sched() can be
replaced by synchronize_rcu().  This commit therefore makes this change,
even though it is but a comment.
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
Cc: Dennis Zhou <dennis@kernel.org>
Cc: Christoph Lameter <cl@linux.com>
Acked-by: Tejun Heo <tj@kernel.org>

include/linux/percpu-rwsem.h

@@ -41,7 +41,7 @@ static inline void percpu_down_read_preempt_disable(struct percpu_rw_semaphore *
 	 * cannot both change sem->state from readers_fast and start checking
 	 * counters while we are here. So if we see !sem->state, we know that
 	 * the writer won't be checking until we're past the preempt_enable()
-	 * and that one the synchronize_sched() is done, the writer will see
+	 * and that once the synchronize_rcu() is done, the writer will see
 	 * anything we did within this RCU-sched read-size critical section.
 	 */
 	__this_cpu_inc(*sem->read_count);