blk-cgroup: move blkcg_{pin,unpin}_online out of line

Move these two functions out of line as there is no good reason
to inline them.  Also switch to passing a cgroup_subsys_state
instead of doing the conversion in the caller to prepare for making
the blkcg structure private to blk-cgroup.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20220420042723.1010598-7-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 7176af6..93cddbc 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -390,7 +390,6 @@ static void cgwb_release_workfn(struct work_struct *work)
 {
 	struct bdi_writeback *wb = container_of(work, struct bdi_writeback,
 						release_work);
-	struct blkcg *blkcg = css_to_blkcg(wb->blkcg_css);
 	struct backing_dev_info *bdi = wb->bdi;
 
 	mutex_lock(&wb->bdi->cgwb_release_mutex);
@@ -401,7 +400,7 @@ static void cgwb_release_workfn(struct work_struct *work)
 	mutex_unlock(&wb->bdi->cgwb_release_mutex);
 
 	/* triggers blkg destruction if no online users left */
-	blkcg_unpin_online(blkcg);
+	blkcg_unpin_online(wb->blkcg_css);
 
 	fprop_local_destroy_percpu(&wb->memcg_completions);
 
@@ -511,7 +510,7 @@ static int cgwb_create(struct backing_dev_info *bdi,
 			list_add_tail_rcu(&wb->bdi_node, &bdi->wb_list);
 			list_add(&wb->memcg_node, memcg_cgwb_list);
 			list_add(&wb->blkcg_node, blkcg_cgwb_list);
-			blkcg_pin_online(blkcg);
+			blkcg_pin_online(blkcg_css);
 			css_get(memcg_css);
 			css_get(blkcg_css);
 		}