net: sk_pacing_shift_update() helper

In commit 3a9b76fd ("tcp: allow drivers to tweak TSQ logic")
I gave a code sample to set sk->sk_pacing_shift that was not complete.

Better add a helper that can be used by drivers without worries,
and maybe amended in the future.

A wifi driver might use it from its ndo_start_xmit()

Following call would setup TCP to allow up to ~8ms of queued data per
flow.

sk_pacing_shift_update(skb->sk, 7);
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sock.h

@@ -2407,4 +2407,15 @@ static inline int sk_get_rmem0(const struct sock *sk, const struct proto *proto)
 	return *proto->sysctl_rmem;
 }
 
+/* Default TCP Small queue budget is ~1 ms of data (1sec >> 10)
+ * Some wifi drivers need to tweak it to get more chunks.
+ * They can use this helper from their ndo_start_xmit()
+ */
+static inline void sk_pacing_shift_update(struct sock *sk, int val)
+{
+	if (!sk || !sk_fullsock(sk) || sk->sk_pacing_shift == val)
+		return;
+	sk->sk_pacing_shift = val;
+}
+
 #endif	/* _SOCK_H */