Bluetooth: hidp: fix device disconnect on idle timeout

Bluetooth: hidp: fix device disconnect on idle timeout

The HIDP specs define an idle-timeout which automatically disconnects a
device. This has always been implemented in the HIDP layer and forced a
synchronous shutdown of the hidp-scheduler. This works just fine, but
lacks a forced disconnect on the underlying l2cap channels. This has been
broken since:

    commit 5205185d461d5902325e457ca80bd421127b7308
    Author: David Herrmann <dh.herrmann@gmail.com>
    Date:   Sat Apr 6 20:28:47 2013 +0200

        Bluetooth: hidp: remove old session-management

The old session-management always forced an l2cap error on the ctrl/intr
channels when shutting down. The new session-management skips this, as we
don't want to enforce channel policy on the caller. In other words, if
user-space removes an HIDP device, the underlying channels (which are
*owned* and *referenced* by user-space) are still left active. User-space
needs to call shutdown(2) or close(2) to release them.

Unfortunately, this does not work with idle-timeouts. There is no way to
signal user-space that the HIDP layer has been stopped. The API simply
does not support any event-passing except for poll(2). Hence, we restore
old behavior and force EUNATCH on the sockets if the HIDP layer is
disconnected due to idle-timeouts (behavior of explicit disconnects
remains unmodified). User-space can still call

    getsockopt(..., SO_ERROR, ...)

..to retrieve the EUNATCH error and clear sk_err. Hence, the channels can
still be re-used (which nobody does so far, though). Therefore, the API
still supports the new behavior, but with this patch it's also compatible
to the old implicit channel shutdown.

Cc: <stable@vger.kernel.org> # 3.10+
Reported-by: Mark Haun <haunma@keteu.org>
Reported-by: Luiz Augusto von Dentz <luiz.dentz@gmail.com>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>