ipv4: tcp: set SOCK_USE_WRITE_QUEUE for ip_send_unicast_reply()

TCP uses per cpu 'sockets' to send some packets :
- RST packets ( tcp_v4_send_reset()) )
- ACK packets for SYN_RECV and TIMEWAIT sockets

By setting SOCK_USE_WRITE_QUEUE flag, we tell sock_wfree()
to not call sk_write_space() since these internal sockets
do not care.

This gives a small performance improvement, merely by allowing
cpu to properly predict the sock_wfree() conditional branch,
and avoiding one atomic operation.
Signed-off-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_ipv4.c

@@ -2384,6 +2384,7 @@ static int __net_init tcp_sk_init(struct net *net)
 					   IPPROTO_TCP, net);
 		if (res)
 			goto fail;
+		sock_set_flag(sk, SOCK_USE_WRITE_QUEUE);
 		*per_cpu_ptr(net->ipv4.tcp_sk, cpu) = sk;
 	}