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>

block/cfq-iosched.c

@@ -1644,14 +1644,20 @@ static void cfq_pd_offline(struct blkg_policy_data *pd)
 	int i;
 
 	for (i = 0; i < IOPRIO_BE_NR; i++) {
-		if (cfqg->async_cfqq[0][i])
+		if (cfqg->async_cfqq[0][i]) {
 			cfq_put_queue(cfqg->async_cfqq[0][i]);
-		if (cfqg->async_cfqq[1][i])
+			cfqg->async_cfqq[0][i] = NULL;
+		}
+		if (cfqg->async_cfqq[1][i]) {
 			cfq_put_queue(cfqg->async_cfqq[1][i]);
+			cfqg->async_cfqq[1][i] = NULL;
+		}
 	}
 
-	if (cfqg->async_idle_cfqq)
+	if (cfqg->async_idle_cfqq) {
 		cfq_put_queue(cfqg->async_idle_cfqq);
+		cfqg->async_idle_cfqq = NULL;
+	}
 
 	/*
 	 * @blkg is going offline and will be ignored by