btrfs: change name and type of private member of btrfs_free_space_ctl

btrfs_free_space_ctl::private is either unset or it always points to
struct btrfs_block_group when it is set. So there's no point in keeping
the unhelpful 'private' name and keeping it an untyped pointer. Change
both the type and name to be self-describing. No functional changes.

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