btrfs: zoned: factor out per-zone logic from btrfs_load_block_group_zone_info

Split out a helper for the body of the per-zone loop in
btrfs_load_block_group_zone_info to make the function easier to read and
modify.

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>