blk-cgroup: fix race between policy activation and blkg destruction

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

blk-cgroup: fix race between policy activation and blkg destruction

When switching an IO scheduler on a block device, blkcg_activate_policy()
allocates blkg_policy_data (pd) for all blkgs attached to the queue.
However, blkcg_activate_policy() may race with concurrent blkcg deletion,
leading to use-after-free and memory leak issues.

The use-after-free occurs in the following race:

T1 (blkcg_activate_policy):
- Successfully allocates pd for blkg1 (loop0->queue, blkcgA)
- Fails to allocate pd for blkg2 (loop0->queue, blkcgB)
- Enters the enomem rollback path to release blkg1 resources

T2 (blkcg deletion):
- blkcgA is deleted concurrently
- blkg1 is freed via blkg_free_workfn()
- blkg1->pd is freed

T1 (continued):
- Rollback path accesses blkg1->pd->online after pd is freed
- Triggers use-after-free

In addition, blkg_free_workfn() frees pd before removing the blkg from
q->blkg_list. This allows blkcg_activate_policy() to allocate a new pd
for a blkg that is being destroyed, leaving the newly allocated pd
unreachable when the blkg is finally freed.

Fix these races by extending blkcg_mutex coverage to serialize
blkcg_activate_policy() rollback and blkg destruction, ensuring pd
lifecycle is synchronized with blkg list visibility.