dm raid: protect md_stop() with 'reconfig_mutex'

__md_stop_writes() and __md_stop() will modify many fields that are
protected by 'reconfig_mutex', and all the callers will grab
'reconfig_mutex' except for md_stop().

Also, update md_stop() to make certain 'reconfig_mutex' is held using
lockdep_assert_held().

Fixes: 9d09e663d550 ("dm: raid456 basic support")
Signed-off-by: Yu Kuai <[e-mail]>
Signed-off-by: Mike Snitzer <[e-mail]>

drivers/md/dm-raid.c

···
 	return 0;
 
 bad_unlock:
-	mddev_unlock(&rs->md);
 	md_stop(&rs->md);
+	mddev_unlock(&rs->md);
 bad:
 	raid_set_free(rs);
 
···
 {
 	struct raid_set *rs = ti->private;
 
+	mddev_lock_nointr(&rs->md);
 	md_stop(&rs->md);
+	mddev_unlock(&rs->md);
 	raid_set_free(rs);
 }
 

drivers/md/md.c

···
 
 void md_stop(struct mddev *mddev)
 {
+	lockdep_assert_held(&mddev->reconfig_mutex);
+
 	/* stop the array and free an attached data structures.
 	 * This is called from dm-raid
 	 */