blkcg: Annotate blkg_hint correctly

commit 55679c8d upstream.

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>
[bwh: Backported to 3.16: adjust filename]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

block/blk-cgroup.h

@@ -47,7 +47,7 @@ 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;
 
 	/* for policies to test whether associated blkcg has changed */