The code is only for blkcg not for all cgroups

Fixes: d4478e92 ("block/loop: make loop cgroup aware")
Reported-by: Nkbuild test robot <fengguang.wu@intel.com>
Signed-off-by: NShaohua Li <shli@fb.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>