null_blk: fix wrong capacity when bs is not 512 bytes

set_capacity() sets device's capacity using 512 bytes sectors.
null_blk calculates the number of sectors by size / bs, which
set_capacity is called with. This led to null_blk exposing the
wrong number of sectors when bs is not 512 bytes.
Signed-off-by: Matias Bjørling <m@bjorling.me>
Reviewed-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

drivers/block/null_blk.c

@@ -589,8 +589,7 @@ static int null_add_dev(void)
 	blk_queue_physical_block_size(nullb->q, bs);
 
 	size = gb * 1024 * 1024 * 1024ULL;
-	sector_div(size, bs);
-	set_capacity(disk, size);
+	set_capacity(disk, size >> 9);
 
 	disk->flags |= GENHD_FL_EXT_DEVT | GENHD_FL_SUPPRESS_PARTITION_INFO;
 	disk->major		= null_major;