net: Use helper macro IP_MAX_MTU in __ip_append_data()

What 0xFFFF means here is actually the max mtu of a ip packet. Use help
macro IP_MAX_MTU here.
Signed-off-by: NMiaohe Lin <linmiaohe@huawei.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/ip_output.c

@@ -996,7 +996,7 @@ static int __ip_append_data(struct sock *sk,
 
 	fragheaderlen = sizeof(struct iphdr) + (opt ? opt->optlen : 0);
 	maxfraglen = ((mtu - fragheaderlen) & ~7) + fragheaderlen;
-	maxnonfragsize = ip_sk_ignore_df(sk) ? 0xFFFF : mtu;
+	maxnonfragsize = ip_sk_ignore_df(sk) ? IP_MAX_MTU : mtu;
 
 	if (cork->length + length > maxnonfragsize - fragheaderlen) {
 		ip_local_error(sk, EMSGSIZE, fl4->daddr, inet->inet_dport,