Input: evdev - properly access RCU-protected 'grab' data

We should use rcu_dereference_protected() when checking if given client
is the one that grabbed the device. This fixes warnings produced by
sparse.
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>

drivers/input/evdev.c

@@ -180,7 +180,10 @@ static int evdev_grab(struct evdev *evdev, struct evdev_client *client)
 
 static int evdev_ungrab(struct evdev *evdev, struct evdev_client *client)
 {
-	if (evdev->grab != client)
+	struct evdev_client *grab = rcu_dereference_protected(evdev->grab,
+					lockdep_is_held(&evdev->mutex));
+
+	if (grab != client)
 		return  -EINVAL;
 
 	rcu_assign_pointer(evdev->grab, NULL);
@@ -259,7 +262,6 @@ static int evdev_release(struct inode *inode, struct file *file)
 	struct evdev *evdev = client->evdev;
 
 	mutex_lock(&evdev->mutex);
-	if (evdev->grab == client)
 	evdev_ungrab(evdev, client);
 	mutex_unlock(&evdev->mutex);