locking/percpu-rwsem: Clean up the lockdep annotations in percpu_down_read()

Based on Peter Zijlstra's earlier patch.

Change percpu_down_read() to use __down_read(), this way we can
do rwsem_acquire_read() unconditionally at the start to make this
code more symmetric and clean.

Originally-From: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

kernel/locking/percpu-rwsem.c

@@ -70,14 +70,14 @@ static bool update_fast_ctr(struct percpu_rw_semaphore *brw, unsigned int val)
 void percpu_down_read(struct percpu_rw_semaphore *brw)
 {
 	might_sleep();
-	if (likely(update_fast_ctr(brw, +1))) {
 	rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 0, _RET_IP_);
+
+	if (likely(update_fast_ctr(brw, +1)))
 		return;
-	}
 
-	down_read(&brw->rw_sem);
+	/* Avoid rwsem_acquire_read() and rwsem_release() */
+	__down_read(&brw->rw_sem);
 	atomic_inc(&brw->slow_read_ctr);
-	/* avoid up_read()->rwsem_release() */
 	__up_read(&brw->rw_sem);
 }
 EXPORT_SYMBOL_GPL(percpu_down_read);