block, bfq: release oom-queue ref to root group on exit

On scheduler init, a reference to the root group, and a reference to
its corresponding blkg are taken for the oom queue. Yet these
references are not released on scheduler exit, which prevents these
objects from be freed. This commit adds the missing reference
releases.
Reported-by: Davide Ferrari <davideferrari8@gmail.com>
Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bfq-iosched.c

@@ -4893,6 +4893,9 @@ static void bfq_exit_queue(struct elevator_queue *e)
 
 	hrtimer_cancel(&bfqd->idle_slice_timer);
 
+	/* release oom-queue reference to root group */
+	bfqg_and_blkg_put(bfqd->root_group);
+
 #ifdef CONFIG_BFQ_GROUP_IOSCHED
 	blkcg_deactivate_policy(bfqd->queue, &blkcg_policy_bfq);
 #else