nbd: use an idr to keep track of nbd devices

To prepare for dynamically adding new nbd devices to the system switch
from using an array for the nbd devices and instead use an idr.  This
copies what loop does for keeping track of its devices.

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