selftests: ublk: fix ublk_find_tgt()

Bounds check for iterator variable `i` is missed, so add it and fix
ublk_find_tgt().

Cc: Johannes Thumshirn <Johannes.Thumshirn@wdc.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250412023035.2649275-2-ming.lei@redhat.com


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