cgroup: call cgroup_subsys->bind on cgroup subsys initialization

Currently, we call cgroup_subsys->bind only on unmount, remount, and
when creating a new root on mount. Since the default hierarchy root is
created in cgroup_init, we will not call cgroup_subsys->bind if the
default hierarchy is freshly mounted. As a result, some controllers will
behave incorrectly (most notably, the "memory" controller will not
enable hierarchy support). Fix this by calling cgroup_subsys->bind right
after initializing a cgroup subsystem.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Signed-off-by: Tejun Heo <tj@kernel.org>