btrfs: Remove fs_info argument from add_new_free_space

This function also takes a btrfs_block_group_cache which contains a
referene to the fs_info. So use that and remove the extra argument.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>