nbd: set discard granularity properly

For some reason we had discard granularity set to 512 always even when
discards were disabled.  Fix this by having the default be 0, and then
if we turn it on set the discard granularity to the blocksize.

Signed-off-by: Josef Bacik <jbacik@fb.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>