Bluetooth: Fix link key persistent storage criteria

Link keys should only be stored if very specific criteria of the
authentication process are fulfilled. This patch essentially copies the
criteria that user space has so far been using to the kernel side so
that the management interface works properly.

Signed-off-by: Johan Hedberg <johan.hedberg@nokia.com>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>