cfq: clear queue pointers from cfqg after unpinning them in cfq_pd_offline
BFQ is already doing a similar thing in its .pd_offline_fn() method
implementation.
While it seems that after commit
4c6994806f70
("blk-throttle: fix race between blkcg_bio_issue_check() and cgroup_rmdir()")
was reverted leaving these pointers intact no longer causes crashes
clearing them is still a sensible thing to do to make the code more robust.
Signed-off-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
Signed-off-by: Jens Axboe <axboe@kernel.dk>