Simplify the code by using kstrndup instead of kzalloc and strncpy in
smk_parse_smack(), which meanwhile remove strncpy as [1] suggests.

[1]: https://github.com/KSPP/linux/issues/90Signed-off-by: NGONG, Ruiqi <gongruiqi1@huawei.com>
Signed-off-by: NCasey Schaufler <casey@schaufler-ca.com>