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>