null_blk: Don't bother validating blocksize

The block queue limits validation does this for us now.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: John Garry <john.g.garry@oracle.com>
Reviewed-by: Zhu Yanjun <yanjun.zhu@linux.dev>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Link: https://lore.kernel.org/r/20240708091651.177447-4-john.g.garry@oracle.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>