UPSTREAM: cgroup: Avoid extra dereference in css_populate_dir()

[ Upstream commit d24f0598 ]

Use css directly instead of dereferencing it from &cgroup->self, while
adding the cgroup v2 cft base and psi files in css_populate_dir(). Both
points to the same css, when css->ss is NULL, this avoids extra
deferences and makes code consistent in usage across the function.
Bug: 391748468
Change-Id: I0c3803cb9833fc94d6ca7b1ddb0b2835a467b997
Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>