blk-cgroup: don't use removal safe list iterators

Commit f1c006f1 moved deletion of the list blkg->q_node from
blkg_destroy() to blkg_free_workfn(). Switch to using the list
iterators, as we don't need removal protection anymore.

Signed-off-by: Daniel Vacek <neelx@redhat.com>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20240104180031.148148-1-neelx@redhat.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>