cgroup: simplify cgroup_load_subsys() failure path

Now that cgroup_unload_subsys() can tell whether the root css is
online or not, we can safely call cgroup_unload_subsys() after idr
init failure in cgroup_load_subsys().

Replace the manual unrolling and invoke cgroup_unload_subsys() on
failure.  This drops cgroup_mutex inbetween but should be safe as the
subsystem will fail try_module_get() and thus can't be mounted
inbetween.  As this means that cgroup_unload_subsys() can be called
before css_sets are rehashed, remove BUG_ON() on %NULL
css_set->subsys[] from cgroup_unload_subsys().

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