blk-cgroup: fix disk reference leak in blkcg_maybe_throttle_current()

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

blk-cgroup: fix disk reference leak in blkcg_maybe_throttle_current()

Add the missing put_disk() on the error path in

blkcg_maybe_throttle_current(). When blkcg lookup, blkg lookup, or

blkg_tryget() fails, the function jumps to the out label which only

calls rcu_read_unlock() but does not release the disk reference acquired

by blkcg_schedule_throttle() via get_device(). Since current->throttle_disk

is already set to NULL before the lookup, blkcg_exit() cannot release

this reference either, causing the disk to never be freed.

Restore the reference release that was present as blk_put_queue() in the

original code but was inadvertently dropped during the conversion from

request_queue to gendisk.