rbd: use DeepCopy() to create thick-provisioned volumes from a snapshot (backport #2184)

[mergify]

This is an automatic backport of pull request #2184 done by Mergify.
Cherry-pick of 7f1bdb4 has failed:

On branch mergify/bp/release-v3.3/pr-2184
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	both modified:   internal/rbd/controllerserver.go

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[nixpanic]

This needs rebasing on top of #2187

[nixpanic]

This won't work on the Ceph version that is used by the release-v3.3 branch.