btrfs: update stale comment for struct btrfs_delayed_ref_node::add_list

The comment refers to a list in the respective delayed ref head that no
longer exists (ref_list), it was replaced with a rbtree (ref_tree) in
commit 0e0adbc ("btrfs: track refs in a rb_tree instead of a list").

So update the stale comment to refer to the rbtree instead of the old
list.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: fdmanana

fs/btrfs/delayed-ref.h

@@ -61,7 +61,8 @@ struct btrfs_delayed_ref_node {
 	/*
 	 * If action is BTRFS_ADD_DELAYED_REF, also link this node to
 	 * ref_head->ref_add_list, then we do not need to iterate the
-	 * whole ref_head->ref_list to find BTRFS_ADD_DELAYED_REF nodes.
+	 * refs rbtree in the corresponding delayed ref head
+	 * (struct btrfs_delayed_ref_head::ref_tree).
 	 */
 	struct list_head add_list;