cgroup: implement cgroup_get_e_css()

Implement cgroup_get_e_css() which finds and gets the effective css
for the specified cgroup and subsystem combination.  This function
always returns a valid pinned css.  This will be used by cgroup
writeback support.

While at it, add comment to cgroup_e_css() to explain why that
function is different from cgroup_get_e_css() and has to test
cgrp->child_subsys_mask instead of cgroup_css(cgrp, ss).

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Zefan Li <lizefan@huawei.com>