chunked: use existing buffer for io.Copy

[giuseppe]

utilize the existing pre-allocated buffer instead of allocating a new one in io.Copy().

[openshift-ci[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: giuseppe

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/containers/storage/blob/main/OWNERS)~~ [giuseppe] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[rhatdan]

LGTM