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.c b/fs/btrfs/async-thread.c
index ff0b0be..e00c8a9 100644
--- a/fs/btrfs/async-thread.c
+++ b/fs/btrfs/async-thread.c
@@ -75,18 +75,18 @@ void btrfs_##name(struct work_struct *arg)				\
 }
 
 struct btrfs_fs_info *
-btrfs_workqueue_owner(struct __btrfs_workqueue *wq)
+btrfs_workqueue_owner(const struct __btrfs_workqueue *wq)
 {
 	return wq->fs_info;
 }
 
 struct btrfs_fs_info *
-btrfs_work_owner(struct btrfs_work *work)
+btrfs_work_owner(const struct btrfs_work *work)
 {
 	return work->wq->fs_info;
 }
 
-bool btrfs_workqueue_normal_congested(struct btrfs_workqueue *wq)
+bool btrfs_workqueue_normal_congested(const struct btrfs_workqueue *wq)
 {
 	/*
 	 * We could compare wq->normal->pending with num_online_cpus()