smack: Replace kzalloc + strncpy with kstrndup

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/90

Signed-off-by: GONG, Ruiqi <[e-mail]>
Signed-off-by: Casey Schaufler <[e-mail]>

security/smack/smack_access.c

···
 	if (i == 0 || i >= SMK_LONGLABEL)
 		return ERR_PTR(-EINVAL);
 
-	smack = kzalloc(i + 1, GFP_NOFS);
-	if (smack == NULL)
+	smack = kstrndup(string, i, GFP_NOFS);
+	if (!smack)
 		return ERR_PTR(-ENOMEM);
-
-	strncpy(smack, string, i);
-
 	return smack;
 }