tcp: define initial scaling factor value as a macro

So that other users could access it. Notably MPTCP will use
it in the next patch.

No functional change intended.

Acked-by: Matthieu Baerts <matttbe@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Link: https://lore.kernel.org/r/20231023-send-net-next-20231023-2-v1-4-9dc60939d371@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/tcp.h

@@ -1489,13 +1489,15 @@ static inline int tcp_space_from_win(const struct sock *sk, int win)
 	return __tcp_space_from_win(tcp_sk(sk)->scaling_ratio, win);
 }
 
+/* Assume a conservative default of 1200 bytes of payload per 4K page.
+ * This may be adjusted later in tcp_measure_rcv_mss().
+ */
+#define TCP_DEFAULT_SCALING_RATIO ((1200 << TCP_RMEM_TO_WIN_SCALE) / \
+				   SKB_TRUESIZE(4096))
+
 static inline void tcp_scaling_ratio_init(struct sock *sk)
 {
-	/* Assume a conservative default of 1200 bytes of payload per 4K page.
-	 * This may be adjusted later in tcp_measure_rcv_mss().
-	 */
-	tcp_sk(sk)->scaling_ratio = (1200 << TCP_RMEM_TO_WIN_SCALE) /
-				    SKB_TRUESIZE(4096);
+	tcp_sk(sk)->scaling_ratio = TCP_DEFAULT_SCALING_RATIO;
 }
 
 /* Note: caller must be prepared to deal with negative returns */