cfq: Clear queue pointers 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>