btrfs: zoned: factor out single bg handling from btrfs_load_block_group_zone_info

Split the code handling a type single block group from
btrfs_load_block_group_zone_info to make the code more readable.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>