md/raid5: use sysfs_notify_dirent_safe to avoid NULL pointer

With the module parameter 'start_dirty_degraded' set,
raid5_spare_active() previously called sysfs_notify_dirent() with a NULL
argument (rdev->sysfs_state) when a rebuild finished.
Signed-off-by: NJonathan Brassow <jbrassow@redhat.com>
Signed-off-by: NMike Snitzer <snitzer@redhat.com>

drivers/md/raid5.c

@@ -5338,7 +5338,7 @@ static int raid5_spare_active(mddev_t *mddev)
 		    && !test_bit(Faulty, &tmp->rdev->flags)
 		    && !test_and_set_bit(In_sync, &tmp->rdev->flags)) {
 			count++;
-			sysfs_notify_dirent(tmp->rdev->sysfs_state);
+			sysfs_notify_dirent_safe(tmp->rdev->sysfs_state);
 		}
 	}
 	spin_lock_irqsave(&conf->device_lock, flags);