blk-cgroup: Drop unlikely before IS_ERR(_OR_NULL)

IS_ERR(_OR_NULL) already contain an 'unlikely' compiler flag and there
is no need to do that again from its callers. Drop it.

Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Jens Axboe <axboe@fb.com>