tcp: Make TCP_MAXSEG minimum more correct.

Use TCP_MIN_MSS instead of constant 64.
Reported-by: NMin Zhang <mzhang@mvista.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp.c

@@ -2246,7 +2246,7 @@ static int do_tcp_setsockopt(struct sock *sk, int level,
 		/* Values greater than interface MTU won't take effect. However
 		 * at the point when this call is done we typically don't yet
 		 * know which interface is going to be used */
-		if (val < 64 || val > MAX_TCP_WINDOW) {
+		if (val < TCP_MIN_MSS || val > MAX_TCP_WINDOW) {
 			err = -EINVAL;
 			break;
 		}