blkcg: Annotate blkg_hint correctly

Avoid that sparse complains about blkg_hint manipulations.

Fixes: a637120e ("blkcg: use radix tree to index blkgs from blkcg")
Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@fb.com>

include/linux/blk-cgroup.h

@@ -45,7 +45,7 @@ struct blkcg {
 	spinlock_t			lock;
 
 	struct radix_tree_root		blkg_tree;
-	struct blkcg_gq			*blkg_hint;
+	struct blkcg_gq	__rcu		*blkg_hint;
 	struct hlist_head		blkg_list;
 
 	struct blkcg_policy_data	*cpd[BLKCG_MAX_POLS];