kernel: cgroup: protect cgroup linked lists

Protecting cgroup lists to fix corruption due to race since
freeing of css_set now in new worker thread, free_css_set_work()

CRs-fixed: 715355
Change-Id: Iec4d7be0c372a17487f96bcdc1542afaff79dd1b
Signed-off-by: Srinivasarao P <spathi@codeaurora.org>