net: tcp: Define the TCP_MAX_WSCALE instead of literal number 14

Define one new macro TCP_MAX_WSCALE instead of literal number '14',
and use U16_MAX instead of 65535 as the max value of TCP window.
There is another minor change, use rounddown(space, mss) instead of
(space / mss) * mss;

Signed-off-by: Gao Feng <fgao@ikuai8.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/tcp.h b/include/net/tcp.h
index 582e377..cc6ae0a 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -78,6 +78,9 @@ void tcp_time_wait(struct sock *sk, int state, int timeo);
 /* Maximal number of ACKs sent quickly to accelerate slow-start. */
 #define TCP_MAX_QUICKACKS	16U
 
+/* Maximal number of window scale according to RFC1323 */
+#define TCP_MAX_WSCALE		14U
+
 /* urg_data states */
 #define TCP_URG_VALID	0x0100
 #define TCP_URG_NOTYET	0x0200