selftests: ublk: don't take same backing file for more than one ublk devices

Don't use same backing file for more than one ublk devices, and avoid
concurrent write on same file from more ublk disks.

Fixes: 8ccebc19 ("selftests: ublk: support UBLK_F_AUTO_BUF_REG")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250623011934.741788-3-ming.lei@redhat.com


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