de-indirect TCP congestion control

[Stephen Hemminger <stephen@networkplumber.org>]

Since indirect calls are expensive, and now even more so, perhaps we should figure out
a way to make the default TCP congestion control hooks into direct calls.
99% of the users just use the single CC module compiled into the kernel.

Use some macros (or other stuff) to change indirect call to direct
calls. So that code like:

static void tcp_cong_avoid(struct sock *sk, u32 ack, u32 acked)
{
	const struct inet_connection_sock *icsk = inet_csk(sk);

	icsk->icsk_ca_ops->cong_avoid(sk, ack, acked);

to:

static void tcp_cong_avoid(struct sock *sk, u32 ack, u32 acked)
{
	const struct tcp_congestion_ops *ops = inet_csk(sk)->icsk_ca_ops;

	if (ops == default_tcp_ops)
		default_tcp_cong_avoid(s, ack, acked);
	else
		icsk->icsk_ca_ops->cong_avoid(sk, ack, acked);


The use macros and/or compiler symbols so the builtin (non-module) congestion
control is always default_tcp_XXX.