ublk: set canceling flag even when disk is not allocated

ublk: set canceling flag even when disk is not allocated

ublk_start_cancel() previously bailed out early when ublk_get_disk()
returned NULL, treating it as "our disk has been dead".  That is correct
for the post-teardown case, but it also wrongly covers the pre-start
case: ublk_ctrl_start_dev() has not assigned ub->ub_disk yet, while
io_uring is already tearing down the daemon's uring_cmds via
ublk_uring_cmd_cancel_fn().

In that window, the cancel path skips ublk_set_canceling(), so
ubq->canceling stays false, even though ublk_cancel_cmd() goes on to
NULL out every io->cmd.  ublk_ctrl_start_dev() then proceeds to set
ub->ub_disk, call add_disk(), and schedule partition_scan_work.  When
ublk_partition_scan_work() runs bdev_disk_changed() and the resulting
read reaches ublk_queue_rq() -> ublk_queue_cmd(), the ubq->canceling
check passes and the code dereferences the NULL io->cmd:

  BUG: kernel NULL pointer dereference, address: 0000000000000018
  RIP: ublk_queue_cmd drivers/block/ublk_drv.c [inline]
  RIP: ublk_queue_rq+0x73/0x100
  Call Trace:
   blk_mq_dispatch_rq_list+0x1c5/0xca0
   ...
   bdev_disk_changed+0x3d4/0x5e0
   ublk_partition_scan_work+0x89/0xe0
   process_one_work+0x344/0x8a0

Fix it by always setting ub->canceling / ubq->canceling under
cancel_mutex.  When the disk is allocated, keep the existing
quiesce/unquiesce dance so the flag is observed across the
ublk_queue_rq() barrier.  When the disk is not yet allocated, there is
no request_queue and ublk_queue_rq() cannot be running concurrently, so
simply flipping the flag is sufficient: any subsequent I/O - including
the partition scan started by ublk_ctrl_start_dev() - will see
canceling set and be aborted via __ublk_queue_rq_common().

Fixes: 7fc4da6a304b ("ublk: scan partition in async way")
Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Link: https://patch.msgid.link/20260527144042.2095194-1-tom.leiming@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>