ublk: fix use-after-free in ublk_partition_scan_work

In the Linux kernel, the following vulnerability has been resolved:

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.