UPSTREAM: blk-cgroup: check for pd_(alloc|free)_fn in blkcg_activate_policy()

Currently all policies implement pd_(alloc|free)_fn, however, this is
not necessary for ioprio that only works for blkcg, not blkg.

There are no functional changes, prepare to cleanup activating ioprio
policy.

Change-Id: I47d38ac673419e9676de6f13838f55f45027d35e
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20240719071506.158075-2-yukuai1@huaweicloud.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>
Bug:427107450
(cherry picked from commit ae8650b4)
Reviewed-by: Zhengxu Zhang <zhengxu.zhang@unisoc.com>