Inconsistency in 'available space' metrics when reserving more than 2GiB

When setting --fs_data_dirs_reserved_bytes and --fs_wal_dir_reserved_bytes flags to values greater than 2GiBytes, the wal_dir_space_available_bytes and data_dirs_space_available_bytes reports incorrect numbers for both Kudu masters and tablet servers. The metrics report much more space than actually available.