ublk: fix use-after-free in ublk_partition_scan_work

ublk: fix use-after-free in ublk_partition_scan_work

A race condition exists between the async partition scan work and device
teardown that can lead to a use-after-free of ub->ub_disk:

1. ublk_ctrl_start_dev() schedules partition_scan_work after add_disk()
2. ublk_stop_dev() calls ublk_stop_dev_unlocked() which does:
   - del_gendisk(ub->ub_disk)
   - ublk_detach_disk() sets ub->ub_disk = NULL
   - put_disk() which may free the disk
3. The worker ublk_partition_scan_work() then dereferences ub->ub_disk
   leading to UAF

Fix this by using ublk_get_disk()/ublk_put_disk() in the worker to hold
a reference to the disk during the partition scan. The spinlock in
ublk_get_disk() synchronizes with ublk_detach_disk() ensuring the worker
either gets a valid reference or sees NULL and exits early.

Also change flush_work() to cancel_work_sync() to avoid running the
partition scan work unnecessarily when the disk is already detached.

Fixes: 7fc4da6a304b ("ublk: scan partition in async way")
Reported-by: Ruikai Peng <ruikai@pwno.io>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>