cgroup: add helper for checking when css is cgroup::self

The cgroup struct has a css field called "self". The main difference
between this css and the others found in the cgroup::subsys array is that
cgroup::self has a NULL subsystem pointer. There are several places where
checks are performed to determine whether the css in question is
cgroup::self or not. Instead of accessing css->ss directly, introduce a
helper function that shows the intent and use where applicable.

Signed-off-by: JP Kobryn <inwardvessel@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>