Bluetooth: notify userspace of security level change

It fixes L2CAP socket based security level elevation during a
connection. The HID profile needs this (for keyboards) and it is the only
way to achieve the security level elevation when using the management
interface to talk to the kernel (hence the management enabling patch
being the one that exposes this issue).

It enables the userspace a security level change when the socket is
already connected and create a way to notify the socket the result of the
request. At the moment of the request the socket is made non writable, if
the request fails the connections closes, otherwise the socket is made
writable again, POLL_OUT is emmited.

Signed-off-by: Gustavo Padovan <gustavo@padovan.org>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>