Now that these two structs are the same, move the btrfs_data_ref and
btrfs_tree_ref up and use these in the btrfs_delayed_ref_node.  Then
remove the btrfs_delayed_*_ref structs.
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>