Bluetooth: ISO: set BT_LISTEN before requesting a BIG sync

A BIS connection is matched to its parent socket by looking for a
socket in BT_LISTEN state with the same BIG handle:

  iso_conn_ready()
    if (test_bit(HCI_CONN_BIG_SYNC, &hcon->flags))
            parent = iso_get_sock(hdev, &hcon->src, &hcon->dst,
                                  BT_LISTEN, iso_match_big_hcon, hcon);

The socket was only moved to BT_LISTEN after iso_conn_big_sync()
returned, while the LE BIG Create Sync command has already been queued
by then. If the BIG sync is established before the state is updated,
which is easy to hit with an emulated controller as the command may
complete in a few hundred microseconds, no parent is found and the BIS
connections are never notified to the listening socket.

The user space is then left waiting for connections that never arrive,
e.g. bluetoothd never completes a MediaTransport1.Acquire of a
Broadcast Sink transport.

Move the socket to BT_LISTEN before requesting the BIG sync, so the
state is visible by the time the command is queued, and restore the
previous state if the request could not be started. Since the socket is
briefly visible as a listening socket, child sockets may have been
queued in the meantime, so drain the accept queue before restoring the
state: the cleanup paths of BT_CONNECT2/BT_CONNECTED don't do it and the
children would be left with a dangling parent pointer.

Fixes: fbdc4bc47268 ("Bluetooth: ISO: Use defer setup to separate PA sync and BIG sync")
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>