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 0e0adbcf ("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>