smack: bugfix the smack label of symlink when '--with-smack-run-label' is set

Even though systemd has its own smack label since
'--with-smack-run-label' configuration is set, the smack label of each
CGROUP root directory should have the star (i.e. *) label. This is
mainly because current Linux Kernel set the label in this way.
(Refer to smack_d_instantiate() in security/smack/smack_lsm.c)

However, if systemd has its own smack label and arg_join_controllers is
explicitly set or initialized by initialize_join_controllers() function,
current systemd creates the symlink in CGROUP root directory with its
own smack label as below.

lrwxrwxrwx. 1 root root System  11 Dec 31 16:00 cpu -> cpu,cpuacct
dr-xr-xr-x. 4 root root *        0 Dec 31 16:01 cpu,cpuacct
lrwxrwxrwx. 1 root root System  11 Dec 31 16:00 cpuacct -> cpu,cpuacct

This patch fixes that bug by copying the smack label from the origin.