TCP 之 TCP_NEW_SYN_RECV状态

TCP_SYN_RECV state is currently used by fast open sockets.

Initial TCP requests (the pseudo sockets created when a SYN is received)
are not yet associated to a state. They are attached to their parent,
and the parent is in TCP_LISTEN state.

This commit adds TCP_NEW_SYN_RECV state, so that we can convert
TCP stack to a different schem gradually.

------------------------
We need to identify request sock when they‘ll be visible in
global ehash table.

ireq_state is an alias to req.__req_common.skc_state.

Its value is set to TCP_NEW_SYN_RECV

 1 struct request_sock *inet_reqsk_alloc(const struct request_sock_ops *ops,
 2                       struct sock *sk_listener,
 3                       bool attach_listener)
 4 {
 5     struct request_sock *req = reqsk_alloc(ops, sk_listener,
 6                            attach_listener);
 7 
 8     if (req) {
 9         struct inet_request_sock *ireq = inet_rsk(req);
10 
11         kmemcheck_annotate_bitfield(ireq, flags);
12         ireq->opt = NULL;
13 #if IS_ENABLED(CONFIG_IPV6)
14         ireq->pktopts = NULL;
15 #endif
16         atomic64_set(&ireq->ir_cookie, 0);
17         ireq->ireq_state = TCP_NEW_SYN_RECV;
18         write_pnet(&ireq->ireq_net, sock_net(sk_listener));
19         ireq->ireq_family = sk_listener->sk_family;
20     }
21 
22     return req;
23 }