kselftests: cgroup: don't fail on cg_kill_all() error in cg_destroy()

If the cgroup destruction races with an exit() of a belonging
process(es), cg_kill_all() may fail. It's not a good reason to make
cg_destroy() fail and leave the cgroup in place, potentially causing
next test runs to fail.

Signed-off-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: kernel-team@fb.com
Cc: linux-kselftest@vger.kernel.org