ublk: add DMA alignment limit

ublk: add DMA alignment limit

The in-tree ublk driver doesn't need DMA alignment limit because there
is one data copy between request pages and the userspace buffer.

However, ublk is going to support zero copy, then DMA alignment limit
is required, because same IO buffer is forwarded to backend which may
have specific buffer DMA alignment limit, so the limit has to be exposed
from the frontend driver to client application.

Cc: Keith Busch <kbusch@kernel.org>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250227103707.2640014-1-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>