linux-phytec-fsl

nbd: add a status netlink command

Allow users to query the status of existing nbd devices.  Right now this
only returns whether or not the device is connected, but could be
extended in the future to include more information.

Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Jens Axboe <axboe@fb.com>