btrfs: export space_info_add_*_bytes

Prep work for consolidating all of the space_info code into one file.
We need to export these so multiple files can use them.

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