btrfs: change return type of cache_save_setup to void
None of the callers of `cache_save_setup` care about the return type as
the space cache is purely and optimization.
Also the free space cache is a deprecated feature that is being phased
out.
Change the return type of `cache_save_setup` to void to reflect this.
Reviewed-by: Filipe Manana <fdmanana@suse.com> Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com> Signed-off-by: David Sterba <dsterba@suse.com>