PM: Lock PM device list mutex in show_dev_hash()

Lock the PM device list mutex using device_pm_lock() and
device_pm_unlock() around the list iteration in show_dev_hash().

show_dev_hash() was reverse iterating dpm_list without first locking the
mutex that the functions in drivers/base/power/main.c lock. I assume
this was unintentional since there is no comment suggesting why the lock
might not be necessary.
Signed-off-by: NJames Hogan <james@albanarts.com>
Signed-off-by: NRafael J. Wysocki <rjw@sisk.pl>

drivers/base/power/trace.c

@@ -188,8 +188,10 @@ static int show_file_hash(unsigned int value)
 static int show_dev_hash(unsigned int value)
 {
 	int match = 0;
-	struct list_head *entry = dpm_list.prev;
+	struct list_head *entry;
 
+	device_pm_lock();
+	entry = dpm_list.prev;
 	while (entry != &dpm_list) {
 		struct device * dev = to_device(entry);
 		unsigned int hash = hash_string(DEVSEED, dev_name(dev), DEVHASH);
@@ -199,6 +201,7 @@ static int show_dev_hash(unsigned int value)
 		}
 		entry = entry->prev;
 	}
+	device_pm_unlock();
 	return match;
 }