netfilter: ctnetlink: remove null test before kfree

Fix checkpatch warning:
WARNING: kfree(NULL) is safe this check is probably not required
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_conntrack_netlink.c

@@ -742,8 +742,7 @@ static int ctnetlink_done(struct netlink_callback *cb)
 {
 	if (cb->args[1])
 		nf_ct_put((struct nf_conn *)cb->args[1]);
-	if (cb->data)
-		kfree(cb->data);
+	kfree(cb->data);
 	return 0;
 }