'nr_undestroyed_grps' in struct throtl_data was used to count
the number of throtl_grp related with throtl_data, but now
throtl_grp is tracked by blkcg_gq, so it is useless anymore.
Signed-off-by: NHou Tao <houtao1@huawei.com>
Signed-off-by: NJens Axboe <axboe@fb.com>