Smack: Label cgroup files for systemd

The cgroup filesystem isn't ready for an LSM to
properly use extented attributes. This patch makes
files created in the cgroup filesystem usable by
a system running Smack and systemd.

Targeted for git://git.gitorious.org/smack-next/kernel.git



Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>