net: Introduce net.ipv4.tcp_migrate_req.

[ Upstream commit f9ac779f881c2ec3d1cdcd7fa9d4f9442bf60e80 ]

This commit adds a new sysctl option: net.ipv4.tcp_migrate_req. If this
option is enabled or eBPF program is attached, we will be able to migrate
child sockets from a listener to another in the same reuseport group after
close() or shutdown() syscalls.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Benjamin Herrenschmidt <benh@amazon.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20210612123224.12525-2-kuniyu@amazon.co.jp
Stable-dep-of: 3a037f0f3c4b ("tcp: annotate data-races around icsk->icsk_syn_retries")
Signed-off-by: Sasha Levin <sashal@kernel.org>

include/net/netns/ipv4.h

@@ -131,6 +131,7 @@ struct netns_ipv4 {
 	u8 sysctl_tcp_syn_retries;
 	u8 sysctl_tcp_synack_retries;
 	u8 sysctl_tcp_syncookies;
+	u8 sysctl_tcp_migrate_req;
 	int sysctl_tcp_reordering;
 	u8 sysctl_tcp_retries1;
 	u8 sysctl_tcp_retries2;