cgroup: make cgroup_write_event_control() use css_from_dir() instead of __d_cgrp()
cgroup: make cgroup_write_event_control() use css_from_dir() instead of __d_cgrp()

cgroup_event will be moved to its only user - memcg.  Replace
__d_cgrp() usage with css_from_dir(), which is already exported.  This
also simplifies the code a bit.

Signed-off-by: Tejun Heo <546b05909706652891a87f7bfe385ae147f61f91@kernel.org>
Acked-by: Li Zefan <a8167b9d27aca9c6624754deb52c4c6420eded04@huawei.com>
Acked-by: Kirill A. Shutemov <eec72676ad9a62f2727dfed6d270d98c0d02adec@linux.intel.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs