Smack: Use memdup_user() rather than duplicating its implementation

Reuse existing functionality from memdup_user() instead of keeping
duplicate source code.

This issue was detected by using the Coccinelle software.
Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>

security/smack/smackfs.c

@@ -2523,14 +2523,9 @@ static ssize_t smk_write_revoke_subj(struct file *file, const char __user *buf,
 	if (count == 0 || count > SMK_LONGLABEL)
 		return -EINVAL;
 
-	data = kzalloc(count, GFP_KERNEL);
-	if (data == NULL)
-		return -ENOMEM;
-
-	if (copy_from_user(data, buf, count) != 0) {
-		rc = -EFAULT;
-		goto out_data;
-	}
+	data = memdup_user(buf, count);
+	if (IS_ERR(data))
+		return PTR_ERR(data);
 
 	cp = smk_parse_smack(data, count);
 	if (IS_ERR(cp)) {