By default, this will be 511, as that's the block layer default. But
drivers these days can support memory alignments that aren't tied to
the sector sizes, instead just being limited by what the DMA engine
supports. An example is NVMe, where it's generally set to a 32-bit or
64-bit boundary. As ublk itself doesn't really care, just set it low
enough that we don't run into issues with NVMe where the required
O_DIRECT memory alignment is now more restrictive on ublk than it is
on the underlying device.

This was triggered by spurious -EINVAL returns on O_DIRECT IO on a
setup with ublk managing NVMe devices, which previously worked just
fine on the NVMe device itself. With the alignment relaxed, the test
works fine.
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>