blk-throttle: Use helper function to add root throtl group to lists

Use same helper function for root group as we use with dynamically
allocated groups to add it to various lists.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 97ea7f8..b9412d1 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -1176,10 +1176,8 @@
 	td->root_tg = tg;
 
 	rcu_read_lock();
-	blkiocg_add_blkio_group(&blkio_root_cgroup, &tg->blkg, (void *)td,
-					0, BLKIO_POLICY_THROTL);
+	throtl_init_add_tg_lists(td, tg, &blkio_root_cgroup);
 	rcu_read_unlock();
-	throtl_add_group_to_td_list(td, tg);
 
 	/* Attach throtl data to request queue */
 	q->td = td;