btrfs: zoned: introduce a zone_info struct in btrfs_load_block_group_zone_info

Add a new zone_info structure to hold per-zone information in
btrfs_load_block_group_zone_info and prepare for breaking out helpers
from it.

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>