ublk_drv: add device parameter UBLK_PARAM_TYPE_DEVT

Userspace side only knows device ID, but the associated path of ublkc* and
ublkb* could be changed by udev, and that depends on userspace's policy, so
add parameter of UBLK_PARAM_TYPE_DEVT for retrieving major/minor of the
ublkc* and ublkb*, then user may figure out major/minor of the ublk disks
he/she owns. With major/minor, it is easy to find the device node path.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20230106041711.914434-5-ming.lei@redhat.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>