nbd: change the default nbd partitions

There's no reason to have partitions disabled for nbd by default, it costs us
nothing to have it enabled and is just confusing/obnoxious to users who try to
use partitions with nbd.

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