nbd: allow device creation at a specific index

If users really want to use a particular index for their nbd device and it
doesn't already exist there's no reason we can't just create it for them.  Do
this instead of erroring out.

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