ublk: detach gendisk from ublk device if add_disk() fails

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

ublk: detach gendisk from ublk device if add_disk() fails

Inside ublk_abort_requests(), gendisk is grabbed for aborting all

inflight requests. And ublk_abort_requests() is called when exiting

the uring context or handling timeout.

If add_disk() fails, the gendisk may have been freed when calling

ublk_abort_requests(), so use-after-free can be caused when getting

disk's reference in ublk_abort_requests().

Fixes the bug by detaching gendisk from ublk device if add_disk() fails.