ublk: don't queue request if the associated uring_cmd is canceled

Commit 524346e9 ("ublk: build batch from IOs in same io_ring_ctx and io task")
need to dereference `io->cmd` for checking if the IO can be added to current
batch, see ublk_belong_to_same_batch() and io_uring_cmd_ctx_handle(). However,
`io->cmd` may become invalid after the uring_cmd is canceled.

Fixes it by only allowing to queue this IO in case that ublk_prep_req()
returns `BLK_STS_OK`, when 'io->cmd' is guaranteed to be valid.

Reported-by: Changhui Zhong <czhong@redhat.com>
Fixes: 524346e9 ("ublk: build batch from IOs in same io_ring_ctx and io task")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250701072325.1458109-1-ming.lei@redhat.com


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