ublk: add UBLK_F_NO_AUTO_PART_SCAN feature flag

ublk: add UBLK_F_NO_AUTO_PART_SCAN feature flag

Add a new feature flag UBLK_F_NO_AUTO_PART_SCAN to allow users to suppress
automatic partition scanning when starting a ublk device.

This is useful for some cases in which user don't want to scan
partitions.

Users still can manually trigger partition scanning later when appropriate
using standard tools (e.g., partprobe, blockdev --rereadpt).

Reported-by: Yoav Cohen <yoav@nvidia.com>
Link: https://lore.kernel.org/linux-block/DM4PR12MB63280C5637917C071C2F0D65A9A8A@DM4PR12MB6328.namprd12.prod.outlook.com/
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>