btrfs: constify tracepoint arguments

Tracepoint arguments are all read-only.  If we mark the arguments
as const, we're able to keep or convert those arguments to const
where appropriate.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/async-thread.h b/fs/btrfs/async-thread.h
index 1f95973..fc957e0 100644
--- a/fs/btrfs/async-thread.h
+++ b/fs/btrfs/async-thread.h
@@ -82,7 +82,7 @@ void btrfs_queue_work(struct btrfs_workqueue *wq,
 void btrfs_destroy_workqueue(struct btrfs_workqueue *wq);
 void btrfs_workqueue_set_max(struct btrfs_workqueue *wq, int max);
 void btrfs_set_work_high_priority(struct btrfs_work *work);
-struct btrfs_fs_info *btrfs_work_owner(struct btrfs_work *work);
-struct btrfs_fs_info *btrfs_workqueue_owner(struct __btrfs_workqueue *wq);
-bool btrfs_workqueue_normal_congested(struct btrfs_workqueue *wq);
+struct btrfs_fs_info *btrfs_work_owner(const struct btrfs_work *work);
+struct btrfs_fs_info *btrfs_workqueue_owner(const struct __btrfs_workqueue *wq);
+bool btrfs_workqueue_normal_congested(const struct btrfs_workqueue *wq);
 #endif