kernel: cgroup: protecting css_set object

Accessing the css_set object should be protected with css_set_lock.
NULL check is required before accesing object.

CRs-fixed: 723762
Change-Id: Ia3a0314a04419889d5002a8f2bf2c1fe9dfa3671
Signed-off-by: Swetha Chikkaboraiah <schikk@codeaurora.org>