6lowpan: add support for getting short address

In case of sending RA messages we need some way to get the short address
from an 802.15.4 6LoWPAN interface. This patch will add a temporary
debugfs entry for experimental userspace api.

Reviewed-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Alexander Aring <aar@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>