netfilter: xt_TCPMSS: Refactor the codes to decrease one condition check and more readable

The origin codes perform two condition checks with dst_mtu(skb_dst(skb))
and in_mtu. And the last statement is "min(dst_mtu(skb_dst(skb)),
in_mtu) - minlen". It may let reader think about how about the result.
Would it be negative.

Now assign the result of min(dst_mtu(skb_dst(skb)), in_mtu) to a new
variable, then only perform one condition check, and it is more readable.
Signed-off-by: Gao Feng <fgao@ikuai8.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/xt_TCPMSS.c

@@ -110,18 +110,14 @@ tcpmss_mangle_packet(struct sk_buff *skb,
 	if (info->mss == XT_TCPMSS_CLAMP_PMTU) {
 		struct net *net = par->net;
 		unsigned int in_mtu = tcpmss_reverse_mtu(net, skb, family);
+		unsigned int min_mtu = min(dst_mtu(skb_dst(skb)), in_mtu);
 
-		if (dst_mtu(skb_dst(skb)) <= minlen) {
+		if (min_mtu <= minlen) {
 			net_err_ratelimited("unknown or invalid path-MTU (%u)\n",
-					    dst_mtu(skb_dst(skb)));
+					    min_mtu);
 			return -1;
 		}
-		if (in_mtu <= minlen) {
-			net_err_ratelimited("unknown or invalid path-MTU (%u)\n",
-					    in_mtu);
-			return -1;
-		}
-		newmss = min(dst_mtu(skb_dst(skb)), in_mtu) - minlen;
+		newmss = min_mtu - minlen;
 	} else
 		newmss = info->mss;