cgroup: reorganize css init / exit paths

css (cgroup_subsys_state) lifetime management is about to be
restructured.  In prepartion, make the following mostly trivial
changes.

* init_cgroup_css() is renamed to init_css() so that it's consistent
  with other css handling functions.

* alloc_css_id(), online_css() and offline_css() updated to take @css
  instead of cgroups and subsys IDs.

This patch doesn't make any functional changes.

v2: v1 merged two for_each_root_subsys() loops in cgroup_create() but
    Li Zefan pointed out that it breaks error path.  Dropped.

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