PM / OPP: Add opp_rcu_lockdep_assert() to _find_device_opp()

_find_device_opp() should be called with rcu-read lock or
dev_opp_list_lock held. Add the opp_rcu_lockdep_assert() check to make
sure caller have taken appropriate locks.

Fix comment over the routine as well.
Suggested-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/base/power/opp/core.c

@@ -81,14 +81,18 @@ static struct device_opp *_managed_opp(const struct device_node *np)
  * Return: pointer to 'struct device_opp' if found, otherwise -ENODEV or
  * -EINVAL based on type of error.
  *
- * Locking: This function must be called under rcu_read_lock(). device_opp
- * is a RCU protected pointer. This means that device_opp is valid as long
- * as we are under RCU lock.
+ * Locking: For readers, this function must be called under rcu_read_lock().
+ * device_opp is a RCU protected pointer, which means that device_opp is valid
+ * as long as we are under RCU lock.
+ *
+ * For Writers, this function must be called with dev_opp_list_lock held.
  */
 struct device_opp *_find_device_opp(struct device *dev)
 {
 	struct device_opp *dev_opp;
 
+	opp_rcu_lockdep_assert();
+
 	if (IS_ERR_OR_NULL(dev)) {
 		pr_err("%s: Invalid parameters\n", __func__);
 		return ERR_PTR(-EINVAL);