ovpn: tcp - don't deref NULL sk_socket member after tcp_close()

ovpn: tcp - don't deref NULL sk_socket member after tcp_close()

When deleting a peer in case of keepalive expiration, the peer is
removed from the OpenVPN hashtable and is temporary inserted in a
"release list" for further processing.

This happens in:
ovpn_peer_keepalive_work()
  unlock_ovpn(release_list)

This processing includes detaching from the socket being used to
talk to this peer, by restoring its original proto and socket
ops/callbacks.

In case of TCP it may happen that, while the peer is sitting in
the release list, userspace decides to close the socket.
This will result in a concurrent execution of:

tcp_close(sk)
  __tcp_close(sk)
    sock_orphan(sk)
      sk_set_socket(sk, NULL)

The last function call will set sk->sk_socket to NULL.

When the releasing routine is resumed, ovpn_tcp_socket_detach()
will attempt to dereference sk->sk_socket to restore its original
ops member. This operation will crash due to sk->sk_socket being NULL.

Fix this race condition by testing-and-accessing
sk->sk_socket atomically under sk->sk_callback_lock.

Link: https://lore.kernel.org/netdev/176996279620.3109699.15382994681575380467@eldamar.lan/
Link: https://github.com/OpenVPN/ovpn-net-next/issues/29
Signed-off-by: Antonio Quartulli <antonio@openvpn.net>
Fixes: 11851cbd60ea ("ovpn: implement TCP transport")
Link: https://patch.msgid.link/20260212213130.11497-1-antonio@openvpn.net
Signed-off-by: Paolo Abeni <pabeni@redhat.com>