gatt: StartNotify is not allowed when device is disconnecting

gatt: StartNotify is not allowed when device is disconnecting

This patch fixed a bluetoothd crash in register_notify_cb(). The
crash is incurred by an exception that under some situation, a
characteristic may be freed when register_notify_cb() is invoked.

When a device is disconnecting, the device interface would hold valid
for a while until the disconnection procedure between the client and
the server is completed. If another process happens to request to start
notification of a characteristic on the disconnecting device, it may
incur a problem. In this case, the client would still send the
StartNotify request since the characteristic object is still valid.
However, the characteristic may be freed soon and become invalid
when the corresponding callback function is invoked later. This
leads to the bluetoothd crash due to the segmentation fault.

To handle the exception, if another process requests to start
notification when the device is disconnecting, it should reject the
request.

Tested on Chrome OS that this patch fixes bluetoothd crash in
register_notify_cb().

Signed-off-by: Anuj Jain <anuj01.jain@samsung.com>
Signed-off-by: Ayush Garg <ayush.garg@samsung.com>