cgroup: update init_css() into init_and_link_css()

init_css() takes the cgroup the new css belongs to as an argument and
initializes the new css's ->cgroup and ->parent pointers but doesn't
acquire the matching reference counts.  After the previous patch,
create_css() puts init_css() and reference acquisition right next to
each other.  Let's move reference acquistion into init_css() and
rename the function to init_and_link_css().  This makes sense and is
easier to follow.  This makes the root csses to hold a reference on
cgrp_dfl_root.cgrp, which is harmless.

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