tcp: Migrate TCP_ESTABLISHED/TCP_SYN_RECV sockets in accept queues.

When we call close() or shutdown() for listening sockets, each child socket
in the accept queue are freed at inet_csk_listen_stop(). If we can get a
new listener by reuseport_migrate_sock() and clone the request by
inet_reqsk_clone(), we try to add it into the new listener's accept queue
by inet_csk_reqsk_queue_add(). If it fails, we have to call __reqsk_free()
to call sock_put() for its listener and free the cloned request.

After putting the full socket into ehash, tcp_v[46]_syn_recv_sock() sets
NULL to ireq_opt/pktopts in struct inet_request_sock, but ipv6_opt can be
non-NULL. So, we have to set NULL to ipv6_opt of the old request to avoid
double free.

Note that we do not update req->rsk_listener and instead clone the req to
migrate because another path may reference the original request. If we
protected it by RCU, we would need to add rcu_read_lock() in many places.

Suggested-by: Martin KaFai Lau <[email protected]>
Signed-off-by: Kuniyuki Iwashima <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Reviewed-by: Eric Dumazet <[email protected]>
Acked-by: Martin KaFai Lau <[email protected]>
Link: https://lore.kernel.org/netdev/[email protected]/
Link: https://lore.kernel.org/bpf/[email protected]

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