cgroup: make css_sets pin the associated cgroups

Currently, css_sets don't pin the associated cgroups.  This is okay as
a cgroup with css_sets associated are not allowed to be removed;
however, to help resource tracking for zombie tasks, this is scheduled
to change such that a cgroup can be removed even when it has css_sets
associated as long as none of them are populated.

To ensure that a cgroup doesn't go away while css_sets are still
associated with it, make each associated css_set hold a reference on
the cgroup if non-root.

v2: Root cgroups are special and shouldn't be ref'd by css_sets.

Signed-off-by: Tejun Heo <tj@kernel.org>