Smack: fix a NULL dereference in wrong smack_import_entry() usage

'commit e774ad68 ("smack: pass error code through pointers")'
made this function return proper error codes instead of NULL. Reflect that.

This is a fix for a NULL dereference introduced in
'commit 21abb1ec ("Smack: IPv6 host labeling")'

echo "$SOME_IPV6_ADDR \"test" > /smack/ipv6host
  (this should return EINVAL, it doesn't)
cat /smack/ipv6host
  (derefences 0x000a)
Signed-off-by: Lukasz Pawelczyk <l.pawelczyk@samsung.com>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>

security/smack/smackfs.c

@@ -1501,8 +1501,8 @@ static ssize_t smk_write_net6addr(struct file *file, const char __user *buf,
 	 */
 	if (smack[0] != '-') {
 		skp = smk_import_entry(smack, 0);
-		if (skp == NULL) {
-			rc = -EINVAL;
+		if (IS_ERR(skp)) {
+			rc = PTR_ERR(skp);
 			goto free_out;
 		}
 	} else {