netfilter: xtables: clean up xt_mac match routine

static bool mac_mt(const struct sk_buff *skb, const struct xt_match_param *par)

{

	const struct xt_mac_info *info = par->matchinfo;

    /* Is mac pointer valid? */

    return skb_mac_header(skb) >= skb->head &&

	   skb_mac_header(skb) + ETH_HLEN <= skb->data

	   /* If so, compare... */

	   && ((!compare_ether_addr(eth_hdr(skb)->h_source, info->srcaddr))

		^ info->invert);

	bool ret;

	if (skb_mac_header(skb) < skb->head)

		return false;

	if (skb_mac_header(skb) + ETH_HLEN > skb->data)

		return false;

	ret  = compare_ether_addr(eth_hdr(skb)->h_source, info->srcaddr) == 0;

	ret ^= info->invert;

	return ret;

}

static struct xt_match mac_mt_reg __read_mostly = {