blkcg: fix missing free on error path of blk_iocost_init()

blk_iocost_init() forgot to free its percpu stat on the error path.
Fix it.

Fixes: 7caa4715

 ("blkcg: implement blk-iocost")
Reported-by: Hillf Danton <hdanton@sina.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>